What's your experience level? How much money do you and your friends have to spend? What kind of racing are you looking to do (besides autocross)?

If you're just getting started, the best thing you can do is just go autocross in whatever car you can get your hands on. The Focus would be a safe bet. Reliable, decent handling, a good car to learn in.

You can also do some track events in the Focus... again, reliable and decent handling. Put some decent tires on it and maybe a set of track pads.

I'd recommend doing it that way simply because you won't disrupt your learning process with having to fix a broken car at the track. Ask anybody who's ever build a race car out of an old beater car, they break!

Of course, if you've got 5 friends who are willing to pitch in $1k each, you could build a LeMons/Chump racer. It'll probably break, too... but you'll have fun.