This Dhampir race is effectively a weaker version of a Drow, so I'll compare it to that.
- Ability Score Increase: The Dhampir gets to choose between the ability score bonuses of the Drow and those of the Dragonborn. This is perfectly in line with existing races.
Stuff the Dhampir gets:
Age & Alignment: Don't really matter from a balance perspective.
Size: The same as everyone else.
Speed: The same as almost everyone else, including the Drow.
Superior Darkvision: The same as the Drow.
Sunlight Sensitivity: The same as the Drow.
Dhampir Magic: The Dhampir gets a 1st level spell 1/day at 1st level, and a 2nd level spell 1/day at 3rd level. The Drow gets a cantrip, a 1st level spell 1/day at 3rd level, and a 2nd level spell 1/day at 5th level. This is the first real difference between the two. The Dhampir gets their racial spells 2 levels earlier, which is obviously stronger up until 5th level when it stops mattering. Not getting a free cantrip goes a fair way towards balancing this out, though.
Vampiric Heritage: I'll come back to this.
- Vampire Weaknesses: If you're incapacitated in a place of rest, you're probably asleep. It's pretty easy to kill someone who's asleep in general, so this doesn't really change much. There might be niche cases where it really comes into play but most of the time it shouldn't matter much.
Languages: Common and one other, just like every other race. The freedom to choose is nice, but hardly imbalancing.
Stuff the Dhampir doesn't get that the Drow does:
- Keen Senses: Free proficiency in the most important skill in the game. This is pretty strong.
Fey Ancestry: Advantage on saving throws against some of the most dangerous spells in the game, and immunity to the Sleep spell. This is also pretty strong.
Trance: Depending on your group and your DM, this might be important, but for most players, it's not going to matter a whole lot.
Drow Weapon Training: This is kind of nice, but most builds that need weapon proficiencies involve a class that gives them, so it's not that nice.
Overall, the Dhampir is probably a weaker version of the Drow. Their racial magic trait is a little better before level 5, but after that it's the same, minus a cantrip. Now for the elephant in the room.
Vampiric Heritage
Whenever you grapple, incapacitate, or restrain a creature, you gain the ability to bite that creature dealing 1d6 piercing damage. Furthermore, you can spend your turn drinking the blood of that creature, causing it to loss [sic] 1d6 necrotic damage. The creature's hit point maximum is reduced by an amount equal to the necrotic damage taken, and you regains [sic] hit points equal to that amount. The reduction lasts until the creature finishes a long rest, it dies if this effect reduces its hit point maximum to 0.
The problem with this is "the ability to bite that creature dealing 1d6 piercing damage". Does this require an action? A bonus action? No action at all? Does it give you the ability to do that when you grapple them, or anytime while you're grappling them? It doesn't really give us anyway to know.
The good news is that it doesn't really matter. Unless it requires no action and is anytime while you're grappling them, this is a pretty weak ability. If it's free when you grapple them, you can grapple, bite, let go, grapple, bite, let go and so on up to 4 times (if you're a Fighter). Except that you're just dealing 1d6 damage, so you'd have been better off attacking normally. Assuming your Strength score is 16 or higher, you'd have been better off punching them!
It only gets worse with other interpretations, too. If it requires an action and you can do it anytime while grappling, you're effectively using a turn to deal 1d6 damage, which is pretty much a joke.
Moving along to the other half of the ability, we have the ability to spend your turn drinking the blood of a creature. It's pretty clear what "spend your turn" means, and it makes for a pretty useless ability. Using your entire turn to deal 1d6 damage and gain 1d6 hit points might be ok at level 1 but it's going to become a joke pretty quickly.
The reducing maximum hit points part is useless because if you do enough damage to kill them with it, they would have died anyway. In general, D&D characters want to kill things, not inconvenience them. It could be used as a way to prevent an enemy regaining hit points, but there's a cantrip that does the job better and more efficiently.
So, overall, the Dhampir is pretty much a weakened version of the Drow race with some unique but useless features.
Large parts of the DMG and MM are now available via the open Systems Reference Document (SRD), which is a free to download 400+ page PDF. There are some sites that have organized this document into a Wiki format which will make it even easier to find the stuff you need.
For example, the Curse of Strahd features a special magic weapon. That weapon is not in the book, but is available in the SRD.
sunblade
Things like Werewolves and Vampire Spawn are also available in the SRD. Overall you should have all of the required material available via the CoS + the SRD.
As an extra note, digital versions of the maps are available for purchase from the artist, Mike Schley. These maps include "player-friendly" and "grid-free" versions which are a step beyond just "scanning the book". There are also some free B&W maps of the region available for free.
Also note that the a quest covering the first 3 levels of CoS was released for free by Wizards. The Death House can be downloaded as a 12-page PDF. Again, larger maps are available from the source above.
UPDATE: I manually walked through the book to double-check for magic items not in the SRD. Here's a list of what I found:
- Elixir of Health (available in PotA supplement)
- Potion of Invulnerability (also available in PotA supplement)
- Daern's Instant Fortress is available as Instant Fortress in SRD
- Rod of the Pact Keeper is not available in any open supplement that I can find. If you substitute with a Wand of the War Mage you should satisfy a similar spot in the party without a major change.
Best Answer
I recently finished a CoS campaign with a small, somewhat oddly-built party. As a module, Curse of Strahd does a pretty good job of keeping the pressure on with very little work from the DM. It's fairly easy to tweak encounters, although as a warning it is also not too hard for the PCs to wander off-track and bite off more than they can chew!
Aarakocra and Flying Players
For your specific example, I think Barovia is a fairly safe place to have a player be an Aarakocra. There are a lot of things you can bring in to make things easier.
Terrain
A lot of the valley is forested, and the forests are described as incredibly dense, twisted, dark, and foggy. Flying in there would likely be difficult if not impossible.
As a side portion of terrain, there are a lot of indoor scenes where flying wouldn't put you meaningfully out of reach.
Enemies
Flying may not put you out of reach of your foes here. While some humanoids might not have a ranged weapon it would be easy enough to add one, and additionally, this is gothic horror with a vampire baddie. There are quite literally endless bats.
Weather
Barovia's weather is miserable. It's always overcast, often rainy, and storms are common. Flying in a rainy windstorm sounds like a great way to let the DM move you every turn. Lightning is a very real thing that I'd totally have intersect with a 150 lb chainmail-wearing bird on its way to the ground.
But really, the number one in my mind is: unless this is formal AL play (since they ban L1 Aarakocra anyways), don't worry about it too much! As this excellent answer on flying PCs explains, it is often a self-limiting feature that you as the DM should be aware of, but not worried about.
General Class Balance
I had a party of largely Unearthed Arcana multiclasses, and found it balanced out pretty well. Honestly, for a group going into a campaign with a known vampire big bad in it, they went pretty light on the Divine magic.
With one exception (see spoiler), I didn't see any combo in-game or during spitballing sessions with my players that would seriously de-fang anything in the game. Be sure to pay attention to action economy and estimated encounter difficulty, and you should be fine.