diablo-3
Using Kanai's Cube, I can upgrade a rare item to a legendary using the correct recipe. If I want to end up with Sebor's Nightmare, do I have to start with a particular type of belt? If so, which type – they all just say "belt"?
In general, how would I know which rare item would be needed to upgrade to a chosen legendary item?
As @yx. mentioned already, gloves are very popular for crafting as good rolls can add CritChance, CritDamage, IAS etc. - if you get all 3 of them, then it can be very profitable. The 6 property Exalted Grand Sovereign Vambraces recipe is extremely expensive, an alternative may be the 5 prop version - now that crafting costs have been reduced for 4/5 prop recipes, you can get double the attempts to get a good item for the same cost. Unfortunately, the 5 prop gloves has also skyrocketed from around 350k to 1m+ since the cost reduction.
Some things to keep in mind when choosing a recipe:
Don't invest in weapons: Very unprofitable to craft these; as they must have a few properties to be worth anything at all. viz., +dmg as well as +%dmg at a minimum, and preferably +CritDmg as well - if not, then it is just vendor food.
Don't go for class specific items: If you follow the previous point, this only applies to wizard hats, demon hunter cloaks etc. The reason is that you need a more specific roll for these, e.g.:
So, what is left? Basically ...
Non-class specific armor is your best bet to get more chances of a useful roll where you can try to recoup costs; while you wait for the amazing combos that earn you millions.
Now, among the various slots which ones to target is affected by awareness of the other choices available for that slot, and the importance of specific affixes. The ones I consider as comparatively worse investments and a reason why, are:
Unless you want to be a dedicated crafter (and are very rich already) the 6 property recipes are out of your reach. Gloves are a popular choice so you can consider 5 prop vambraces; unsure of the market for rare chest armor or bracers but they might be a cheaper alternative.
I can now attest first hand that set items are possible, got a Uliana's Strength upgrading a rare shoulder piece with my monk.
With these 2 reddit's threads confirming this also, I'm confident enough to say that the items upgraded will all be for the class they are being upgraded with. Else, set items would not be 100% sure of being for the class upgrading the items which would seem strange.
That is also consistent with crafted items that have stats for the class crafting them.
But, the class-only items will roll with their class stats. I tested with a voodoo mask and got 
Best Answer
You can't upgrade rare items to a specific legendary item. What you get by upgrading is a random legendary of the same type, e.g if you upgrade a rare belt, you get a legendary belt; if you upgrade a rare helm, you get a legendary helm; if you upgrade a rare spirit stone, you get a legendary spirit stone, etc.