Peter:
You will still need torsion tube (at least the last piece near plate-bushings) beacuse most of load should be chanelled trough torsion tube and not four bolts holding the uniball in place.
With other words, the cylindrical insert jammed in torsion-tube should take most of the loads and four bolts should only keep it in place.
So in case you plan to replace torsion-bars with couilovers, make sure you have a healthy portion of tube left.
I don't quite understand what you mean when you ask "the likely cost of coilovers". Do you mean the coils themselves, coils together with shocks or whole assembly?
There are complete kits available at TRG:
http://www.theracersgroup.com/parts/suspension/trg_complete_suspension_kits/trg_complete_suspension_kits.htm
, but they aren't cheap.
We are going to use modified Bilstein Club Sport shocks with threaded pipe and home-made uniball pickup-points so there are no part numbers :-) Springs themselves can be bought in various lengths and ratings.