First you would probably want something that will run the majority of the software.. so that would be either XL or XE. Theyre getting harder to find. I recommended the XL series as probably the first thing you would want to do is drop a U1MB into it, and it is just much easier to do that on an XL (most of them have the chips socketed). After that, probably an SIO2SD of some sort to allow you to take advantage of all the software available on the internet. Next addition would probably be a SIDE2, Then a 800, and drop an Incognito (if you can get one) into that. At this point, you may want to round out your collection with 1200XL and 600XL, both of which have unique features that would add to your collection. Just my opinion, of course
And then it's off to the ST line...