I'd say that in general you're going to want a shutter that has been originally paired with a lens for what Rick says, having the f/numbers line up. Then again, you could save money by getting them separately, and figuring out the f/stops wouldnt be too hard. There are some tricks there.
As for the diameters, I'm not an expert but there are different designations like Copal-1, Copal-0. Though I'm not sure if those refer to the diameter of the threads in a shutter or the hole necessary in the lens board. Either way, there'd be a correlation there.
Good luck!
A good cheap lens would be a Raptar or a Ektar press lens. They're old and were used on press cameras, but they provide some movement latitude and are pretty cool lenses IMO. Strangely, my Wollensack Raptar in a Rapax shutter functions perfectly and has done so since I got it from a community-college darkroom box that had been completely neglected. Meanwhile, my Synchro-Compur that came from a professional photographer has been sent for CLA once and is in need for another now. Of course, that's just one man's experience.
