I agree that getting a good tower is not cheap. Some older towers such as the EZ Mk5 are not suitable for Mediator, having been designed with ZIV in mind (or a PC MOBO). This can make life real hard. Converting a PC tower yourself is OK but tends not to be as tidy as a made to measure Amiga tower.
My advice, keep a close eye on ebay. I got an elbox/winner tower for €127 with 1200MOBO, fastIDE, silver surfer, CDROM and 2 (albeit, small) HDD's.
Not bad eh?
And keep an eye on that doctorq fella, he's allways selling stuff and is a genuine easy bloke to deal with. ;-)