Three years in the past my character and the rest of the party were level 1. At that time, our selfs from 30 years in the future traveled through time and possessed us for two years. We don't remember anything from that period and the plot began one year ago. The spell to go back in time is a continuous spell that is broken when the caster dies or is about to die. My self is responsible of that spell through a dragonmark alteration (travel through time instead of teleporting through space). The first thing that we realized when we got back to our senses is that we were about to be executed, so we assume my self from the future confronted with unavoidable death decided to release the spell.
In 2 weeks, my character and the rest of the party (we are 12th level now) are taking the very same trip through time to the same point in the past three years ago (for several reasons I can explain if you ask). Our goal is to alter the past and survive three years until the moment the spell is cast, at which point I will be able to release the spell. Once again, only our spirits will travel and will possess our past selfs. As a consequence, we'll arrive at the destination with the gear of our past selfs. Only intelligent items that already existed back in that time can take the travel too. The DM's said that this is because the items we'd bring with us haven't yet been created and would cease to exist during the course of the trip. As for non intelligent items, they can't benefit from the spell. (Our characters' research shows this to be true. The DM's not just messing with us.)
In anticipation of this, my swashbuckling-type PC has liquidated all his gear except the clothes on his back and ended up with 100,000 gp. I need to spend this money in such a way that whatever I spend it on will endure the trip intact, and I'd prefer to spend this money on stuff that will improve my character's ability to survive and thrive during our time travel adventure (because if I die or is about to die, it will break the spell).
The 5th-level Sor/Wiz spell permanency [univ] (PH 259-60) can provide limited benefits, but my character's not a caster with only a few ranks in the skill Use Magic Device, so effects that might be extremely valuable are unavailable. (Thus, for example, while my character can get a permanent resistance effect, he can't get a permanent darkvision effect.)
Spellcasting and services can be purchased at the normal rates (PH 129), but spells and effects like gate [conj] (PH 234), lesser planar ally [conj] (PH 261), lesser planar binding [conj] (PH 261-2), and wish [univ] (PH 302-3) are unavailable.
In the present, taking no more than 2 weeks, what can my character do, get done, or buy for 100,000 gp that will improve his ability to survive when he arrives–naked and gearless–in the past?
Restating my assumptions:
Let's start with solving for the trivial case:
We'll call this iteration 0.
You want to survive for three years. Taking that literally, the specification is for three sidereal years, not three years subjective time. You'll want to, this iteration, learn the precise tuning forks required for two demiplanes. Demiplane 1 has the flowing time attribute, and is preferably a nice garden or something. This demiplane has a very very high time ratio, let us say 1 second to 1 hour. (With that ratio, you'll want to spend 7.30484397 hours there for three sidereal years to pass.)
Demiplane 2 has the same flowing time attribute, but in reverse to whatever extreme ratio is allowed.
Hire a caster and manifester to do the following things (I am presuming your party can't do them.):
Manifest Hypercognition (three times) on top of the being's Knowledge (Planes) to find you your two tuning forks. 3600 gp. Call it 400 gp tip (for use of high knowledge skill plus whatever buffs they want to throw in.). 3000 gp. The third cast is to find a nest of Silthilar.
Make sure the caster who can craft wondrous items knows plane shift, craft contingent spell, last breath, and distilled joy. Bring extra casters if necessary.
Plane shift into your low-ratio demiplane (the one that gives you plenty of time). Either have the above spells enchanted into whatever spirit-stuff (I suggest calling them "spirit tattoos") will be sent-back, but fluff it however. Or have as many crafted contingent spells layered into your soul as you can afford. Given that magical effects carry backwards through time, the soul can be affected by magic. If necessary, plan elaborate scry & teleport robberies to pay the caster. You'll have plenty of time, no consequences, and whatever money you can acquire from a world that won't exist.
During this time, conduct interplanar negotiations with the nest of Silthilar. Offer for them to transport whatever research they want back in time for three years in exchange for free grafts on the other end.
Propose to your DM to just use Continuum time travel rules for this, as they're far more nuanced and coherent. Sigh when he/she refuses.
Spend the rest of the time engaged in whatever extra soul-enchantments are allowed by the remainder of your resources.
Go back in time 3 years. Let the first crafted contingent spell take effect and immediately port you into demiplane 1, with the high time ratio. Enjoy a nice book for 8 hours. Let the wibbly time spell elapse.
Swap over to demiplane 2. Spend as much time as necessary (reincarnating via last breath) to figure out how you did the time travel alteration in the first place. Cheese out knowledge checks as required. Then improve the time travel alteration to be an inherent property of your dragonmark. Get whatever other contingent spells/wonderous-soul-item thingies enchanted into your soul. Go back in time 3 years.
Win. you'll have detailed knowledge and research for the next three years of game time. You're in a time-loops scenario. Against any "leveller" (person who's boringly travelling level in time), you will win. When you encounter an unexpected threat, abort universe and iterate again.
Preserved as generic time travel advise:
The lack of items created yet poses a problem, as that criterion applies to your bodies as well. Still, given that something is moving through time (or, you're getting reassembled through information transmitted backwards.)
First, investigate how the Timeless trait interacts with your specific time travel rules. You might just be able to stick your stuff on an appropriate demiplane, remember its specific tuning-fork frequency, and retrieve on the other end.
Second, if stuff, but only your body stuff is moving, then you can get custom enchanted magic items which are slotless, embedded into your flesh. Which custom magic items is left as a negotiation with your DM, but there are an awful lot of first level spells that are absolutely awesome to be enchanted into magical items.
If only contiguous self-organic stuff is moving, look into grafts. While grueseome, get as many of these as your DM will allow. Some of the more risky routes can certainly be attempted within two weeks and can trivially consume all available resources. For a buckle-swasher, absolutely go attempt to contact the Silthilar, promise them to take all of their research back in time in exchange for free grafts, and take as many as they'll perform. If they require money, 100k won't even approach being enough. Still, the money is enough to find them. Otherwise, buy as many fiendish grafts as you can, and just make sure your cleric has lesser restoration prepared.
If only information is being transmitted back, perhaps you may want to start exploring grafting on Continuum to your game. In this instance, invest in crafted contingent spells up to the limit that your DM allows such that you can fake being prepared for your first dungeon crawl. Spend the rest on legend lore and other historical magics so you know what recently-dug grave to knock over to get a reasonable facsimile of your stuff back.
If only information is being transmitted back and your DM isn't being nice about finding a lair via legend lore and contingent spells aren't allowed... Go back further and spend an arbitrary amount of time with liquid pain + restoration powering your caster's recreation of magical items via torture. It'll take a while, but by definition you have as much time as you need. Consider instead ambrosia or any of the other techniques in this answer. Since you're going forearmed with knowledge, you can skip much of the initial bootstrapping, choose somewhere where you wont be bothered for a significant duration of time, up to and including specially found demi-planes for this purpose. Given that you have significant awareness of the past, finding a community that can support your intentions should be trivial.
If your DM isn't okay with self-bootstrapping via strategic information transfer leading to effective immortality, ask him/her what you're allowed to do, and do that. Given timetravel, there exist two possible time machines: the initial prototype and the best possible version. Given information transferring timetravel, there is nothing preventing you from heading back in time, porting to the appropriate location, ensuring you have enough resources for eternal life, spending as many years as it takes on perfecting the time travel ritual, and then heading back again to do things correctly.