During the game quite a few citizens disappear: Umbacano, Agarmir etc. Can someone make a mod that allows me to buy these houses and use them.to live in or let? Seems a real waste to me to leave such gorgeous houses without owner...

I'd love to be able to get Umbacano's mansion after the questline, esp. with OOO so you'd have those Golem guards etc. Anyway, you can use Claim that interior for empty houses - doesn't work for Umbacano's, though, as it has NPCs who will see it as theft.

There's a Fallout 3 mod called "http://www.fallout3nexus.com/downloads/file.php?id=1516" which does basically this. It adds "property manager" books in many interiors which allow you to buy the homes and (optionally) rent them out. It also automatically grants you ownership of some properties when you kill their owners. Given that this is probably done entirely with scripted objects (the property manager books) and on-death events, I expect that it should be possible to do basically the same thing in Oblivion. It's just a question of whether anyone actually has done it yet...

So, is it possible to add an NPC to, let's say Imperial Office of Commerce and the moment a house owner is dead their house an be purchased by player... possible?

training cattle allows vampires to enslave NPCs, and get a number of benefits from them. One of which is that once they like you enough you can ask them to give you their house.