I'm with barythrin: keep a C= system for nostalgia; in my experience, you'll regret getting rid of all of them at a later time, and will just reacquire them again - usually at a much higher expense. Amiga is also really nice, though I wouldn't necessarily delve into the classic PC world unless that's your jazz... you can assuage that desire with DOSBox in most cases, I think, as most of us probably want old computers to experience the games and stuff we loved (or wanted to love) as kids.
I think that the Tandy systems are interesting... which is why I got a 1000HX. Tandy sound, 3.5" disks, DOS in ROM, 640kb, and DMA for an XT-IDE and hard drive. Tandy CoCo units are also pretty cool, and different enough that they'll keep you entertained for awhile, especially if you bite the bullet and get a 512kb RAM expansion and start delving into OS-9 (something I've not yet done, though my CoCo III does have 512kb)
Apple IIgs is a surprisingly nice machine as well (begrudingly admits the long-time Apple-hater). Though be prepared to spend > $100 if you want any form of hard drive solution. Especially since reactivemicro has closed its doors.
Chromedome45 has a nice Apple IIgs for sale, and CoCo's are available all over the place, though be sure to watch pricing carefully... sometimes the bidding can get quite high. Other times, you'll get them for a song. Be aware that the TRS-80 external disk drives will work with the CoCo machines just fine - you'll need the FD-502 disk controller interface, however.