Depends on what you want to play. If you are going to get an MSX, Get a 2+ or Turbo-R. With a 2+ you can play almost everything. Turbo-R adds a few cool extra doujin games though such as Multi-Plex. I like Panasonic machines myself (the only maker of Turbo-Rs), but their 2+ machines WX and WSX (WSX adds S-Video), are quite sexy looking. Sony made some great 2+ machines too.
Just be sure you get a unit with a working floppy drive.
Game wise? Complete MSX games (especially cart games) is the NeoGeo AES of the Japanese Computer world, online prices have gotten quite stupid, a lot of cart games have been hacked into floppy disk images though that you can write easily.
FM Towns is not worth it for most people imo. I quite like it, but you really need to take a good look at what you get before going the Towns route. A lot of fun to be had, but rather impractical in many ways, and a lot of oddities to deal with.
Now X68000 on the other hand is a must have imo. The hardware is quite pricey, but once you get a good working setup, you are golden. It's actually the cheapest of the 3 to collect original games for (in Japan anyway). And if you get a Compact Flash setup, you can run 90% of the games straight off the flash, and the rest can be written to disk. There are only a few games left that require original floppies to play.
Stronger then your average deadite