I think if you were to go wizard (i.e. necromancer) I'll allow the necropolitan with an xp penalty of 1,000 , but if you with DN I'll need to impose a 2,000xp cost to the process. Reason is, DN get the ability to heal themselves at will if they get this template (or Tomb-tainted soul feat), wizards don't. There are enough new benefits that are given (immunity to crits, immunity to Fort saving effects, ability score damage, darkvision, etc.), reflecting the cost.
I hope this doesn't affect your character creation, especially as this adventure offers tons of xp and you'll be to level 6 within a few battles (I'd expect).