Mechanics and Fixing Any Questions Answered

A friend of mine has asked me to help her reinstall the gears on her bike. Apparently she had a nice campagnolo RD stolen along with the downtube shifters. She took it to her lbs at the time (several years ago) and they gave the option of buying spenny new parts, or converting to a ghetto single speed, and she chose the latter. The mechanic apparently said that doing it this way would mean she can convert back to geared later without having to buy a new cassette.

I reckon I could do this for her but wanted to consult the wise people of this forum before I get started:

• Is it possible to reuse this cassette? If so, how do I reassemble it with the spacers in the correct place? Any pointers to some useful online literature would be most helpful.
  
• Given that she's happy staying with downtube friction shifters, am I right in thinking I could fit a shimano RD (given that they seem to be cheaper and more available) with no problems?
  
  

Cheers