Older frames should have a decent amount of dropout so it will be easy enough to set up your drivetrain. The "horizontal" dropouts are better if you want to use fenders.

Older frames with horizontal dropouts will be cheaper than an equivalent frame with track fork ends because of supply and demand.

I've got both, have built up both and it's not a big deal except for the fenders and that's if you get a flat on the rear tire.

Sheldon has this to say: Fixed Gear Conversions