I'm a shade tree mechanic with an old car that's got a fair amount of rust on various fasteners (particularly in the under-regions). When I'm doing maintenance, I often like -- or find the need to -- replace those rusted bits with new pieces. The problem that I run into is that I don't know for sure what I need to buy until I take the thing apart and can look at the fasteners in detail. It would make life a lot more pleasant if I could have the things I needed on hand before disassembly.
I've noticed some correlation between dimension and torque as specified in my FSM -- in fact, there's a table at the beginning that gives the torques for bolts by diameter/pitch and grade -- but there's nothing like "bolt 35 in this diagram is 12mmx1.25" (because of course shops just have bolt bins), and not every bolt has a torque spec. Nor does the Haynes manual have specific info.
Can I rely on the torque/size relationship? What should I do for fasteners with no torque spec? Is there another measurement, resource, trick, or rule of thumb that I'm not aware of that will allow me to be reasonably confident of purchasing the right material? Or should I just resign myself to afternoon bike rides to the Fastenal store?