Keep an eye out for the condition of the cab, and the style too. You will be using this thing, so you want the overall look to be one that you can look at every day.
Make sure you look near the edges of the cab, and pay attention to any damage - chipping, water damage, holes - this is all stuff that will take time to repair.
And like it was stated above, look for something that will fit your ideal control panel. Some cabs have "wings", in between which the CP size is fixed. These tend to only support a fixed width CP unless you plan on cutting the wings off (more work). Other cabs have bolt on CPs, which may be best for bolting on a new, larger CP.
Unless the cab is somewhat unique, you will most likely be replacing the controls. Although some controls are hard to find (e.g. Star Wars yoke or Tron joystick), buttons and basic joysticks will most likely be replaced.
Oh, and finally, while it may seem that it is much less work to convert a cab than to start from scratch, I have found that the conversion is a lot of work too! Considering all the retro fitting, repairing and supports you will be adding, it turns out to be quite a bit of work.
Keep us posted!