What kind of old 10spd? Is the frame worth putting alot of work and new parts into?
When you say fixie/free conversion do you mean a fixed gear and single speed on the same hub? The the first thing you need is new wheels. That opens questions about dropout spacing and brake reach.
An that is just for starters. Any pics of the bike in question?
__________________
“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk