Convert a bike that is a mongrel or missing drivetrain parts. Don't convert a complete, original, perfectly functional, and nice-looking bike like that.
On the other hand, converting it won't ruin it. Save the parts and you can put it back to original. I love riding fixed gear, it's not "style thing", you ride differently when you only have one gear, I find myself thinking about terrian, timing traffic etc. plus it makes you work harder which is one of the reasons I grab the fixed gear so often ( see Doc. I'm listening)
Anyway, I wouldn't have a fixed or SS for my only ride, while they are fun and have certian advantages...gears have some serious functional advantages too. Sounds like youre gonna be needing two bikes...huh?