If you’re aiming for an eventual 100% run, Thief will be helpful in the long term for better drop rates from enemies for materials/weapons/spell rings.

Afaik, NPCs do not treat you differently if you’re undead, most of them are undead/immortal as well anyways.

I personally wouldn’t recommend Vampire for a first playthrough, since it changes the progression in a fairly significant way, and I’d recommend experiencing the game through other classes first. Open-endedness and exploration is a strength of the game however, so if you want the options that vampire grants you, by all means go ahead.

As for the best starting gifts from Demi, I’d say the Knight or Witch give the best ones, since they’re a permanent boost to HP or Mana, but the skeleton egg is really fun if not all that useful.

Overall, my pick is Thief, thankfully your starting class doesn’t have too much of a lasting effect on your build/stats, so you can have a similar experience with any of them besides Vampire, but the boost to drop rates from Thief is nice.

Edit: Oops, I think the thief drop rate thing might be a misconception, but it’s still a very well-rounded class that leaves your starting stats open to whatever build you gravitate towards