So far just three for me, though I've bought/melted a few things to get there.
M50: Bought it entirely cause it looks incredibly sexy, have not felt remotely bad about this purchase yet.
Avenger: I wanted something that was a one seater but had a bed and some cargo room so I could have some sort of capability to do missions solo. Originally I got the 325a, but after staring at the Avenger for awhile I was eventually won over by the massive gun and massive engine. I was also partially motivated by the fact that I kind of suck at dogfighting, and the likely "hit and run" tactics of the Avenger appealed to me. Also it looks awesome.
Redeemer: Originally I bought a Constellation, but when the Redeemer came out I couldn't help but melt the Connie to grab a Redeemer. I love the look of it (people can hate on the hair straighteners all they want, but I think they're neat), and the fact that it had dual manned turrets like the Constellation but in a smaller, more maneuverable package really appealed to me. THEN they added more guns, and I became even happier with my purchase. I'm holding out hope that it becomes possible to ditch the Marine seats in favor of a little extra cargo space, but even if not I think the sheer amount of firepower this thing has now will keep me happy. If it turns out that the Redeemer is truly useless outside of limited situations (as many people seem to be claiming), I'll probably wind up flipping the Redeemer back for a Connie (unless something else has come out by then that I decide I want more).
Not sure what I'll be looking to get in the future. I pretty much refuse to get a ship that I don't like the look of, so it's hard for me to say that I'll want any of the things that haven't been revealed yet.