Cyclocross - CX and SSCX races and training

have been thinking of giving cross a go this winter to keep up racing etc. I don't really have the money buy a proper bike - so I was thinking of trying to get hold of an old rigid mountain bike and giving it a little tlc. Bad idea? I don't expect to be very competitive anyway, so weight etc doesn't really bother me.

It's not a bad idea, but in most conditions, a cross bike is much faster than a mountain bike. Plus you can use old road components on the cross bike as you wear them out and upgrade which is a bit more difficult with a mtb.

You can pick up cheap cyclocross frames and bikes fairly readily.