Hi

First, soak all bolts, fixings etc etc in WD40 for as long as you can before
removal and all should come undone easily!!!
I replaced my subframe not so long back, it took me a weekend but thats only
because while i had the old one off i cleaned back the underside of the boot
floor, the rear valance, the heel board and battery box and coated with
several coats of hammerite and then underseal.
In itself it is not such a daunting task, the haynes manual combined with
some common sense should see you through.
Whatever you replace it with should however be painted with some decent
paint, personally i used hammerite spray cans and gave the new subframe a
couple of good coats.
I cleaned up all the parts that came off the old subframe to ease refitting
and checked them for wear and i replace most (not suspension or brake
components) fixings with stainless steel you can get this from
www.gtc-direct.com
You will need an assistant to support the subframe while removing/refitting,
and i would replace the subframe mounting kit too.
If you are doing a quick job, like for like then you could get it done in
about 4 hours.

Hope that helps
Andy