Chup1n, you can always return from Feedom Islands back to Darion - Where is Greewort, Arlania, Marschan Swamps with many quests and dungeons which will help you to get to higher level and get better items very easily. I strongly advise that.
BTW- What Undead units you can hire is totally RANDOM - once vamps, then zombies, then ghosts... it depends all on luck
But you can't hire necromancers nor Dark Knights in Marshan swamps, maybe in Taron Mines (very rarely) in Elven Lands (in underground - always) in Demonia (very often and very bride variety of undead) and in the last location... no matter what it is called - you will know when you are there (don't want to spoil the game totally for ya)