I'm getting all the bits for my x1000 tomorrow
I would recommend a second hand Sam 440 first, though, as they're cheap and give a good idea of the OS. They're plenty fast enough for most uses, but too slow for hi-def video and the browsers are a bit sluggish... but if you find you like it you can then sell that without any loss, probably, and go up to the next level like a 460, or even an x1000 which is very expensive but very nice! And if you don't like it, you can still sell it and not lose anything.
+1, except I would recommend getting a Sam460 (aka AmigaONE 500) since it supports the latest HD cards and has more grunt for E-UAE stuff ie. heavier AGA games (mind you with the way Rachi is going with JIT this might not be an issue any more).
which ever OS4.x machine you get, I highly recommend getting a Catweasel to use all your old floppies using spirantho's awesome drivers here
http://www.retroreview.com/iang/Catweasel/