I'd consider fit first if you want to spin. Your not going to spin cranks that are too long no mater how fit you are and I recently improved my spinning ability by moving my seat back with a larger rear ofset set post.

See, http://sheldonbrown.com/frame-sizing.html and http://www.cranklength.info/

If your fit is right then riding fixed wheel can be fun in a kind of mad, semi-suicidal way! Going down hill anyway. You have to keep spinning because there's no way your going to use any backpedaling at full rpm without going over the handlebars. Fitting brakes is useful!

You can use a bolt on trainer if you have one with a fixed magnetic resistance setup for spinning anyway.

Regards, Anthony