Palm Vx - No expansion port, looks nice but expensive. 8 Mb
Palm IIIxe - No expansion port. Good price. 8 Mb
Palm VIIx - don't even think about it.
Visor Deluxe - Has expansion port, slightly older Palm OS (like a cross between 3.1 and 3.3), and 8 Mb. Good price. See my sig for more info on the Visor.
Buddha, I dunno where you got that bit about the IIIxe expansion port. If you're talking about the Hotsync connector, the Vx and the IIIxe both have it, but the design is different so accessories made for one won't fit the other. If you want the most compatibility for add-ons overall, probably the IIIxe is the best, although some of the more expensive accessories end up showing up for the Vx first. If you want the most versatility, go for the Visor Deluxe, since the expansion port is quickly becoming supported by many developers. The biggest reason I like the Visor Deluxe is because you can buy a backup module for it. Contrary to popular belief, all of these machines can crash (but less than Windows CE machines), and sometimes it takes all your data with it. It's OK, if you're always near your main computer, but otherwise you're SOL if your Palm goes down.