I had a peugeot that I converted to a fixed as well. You want to make sure first off that your bottom bracket and headset are in good condition. Replacing these things on an old french bike is a pain in the butt. Also, if you are going to replace the stem/handlebar or seatpost, you are going to have to either shim or sand the stem down to accomodate the 22mm opening of a french headtube.
I had a suicide hub. I took my existing wheel and redished it. If you are lucky, the spokes won't bottom out and you will have a pretty centered wheel. I think rotafixed a fixed cog....and never needed a lockring of any kind.

If you are located in boston, there is a guy in framingham that sells cheap fixed rear wheels for like $35.
PM me if you have more questions.