The key is finding one that feels right for you. You can modify anything to suit your needs from there. People have even modified Sportsters for more touring comfort, though that's probably going from a bike meant for one extreme to the polar opposite of that.
If you're looking used, gotta feel the difference between Softail and Dyna, FX and FL, the "small" big twins vs the touring bikes and even the difference between a Road King and a full bagger like an SG or RG. Take a Sportster for a ride and see how that feels.
Of course, everyone knows the best is a red Softail Custom/Standard. With $8,000 to spend, you can't go wrong with a low miles, used Evo big twin. You can get one in running condition from $4-6,000 and still have at least $2,000 to spend on modifications. Plus, if you plan to do your own wrenching, it doesn't get much easier and simpler than an Evo. Though, I might also be a little biased...