Hi finchy sorry to hear about your gearbox issues, you might want to have a word with the person you bought it from as these sort of problems generally don't just appear without warning unless you've tried some full blooded launch and damaged something that way. I also wouldn't write off the clutch being the source of the problem if it simply wont come out of 3rd. It could even be the clutch slave cylinder which I understand can be a weak point on R's, check that that the fluid is at the correct level and that it hasn't lost pressure that way, as if that goes then you would probably not be able to release the gear. Your more likely to have a spontanious problem with the clutch and clutch master cylinder then you are the gearbox so might want to bare that in mind before shelling out for a new box.
With regards to lowering the car 60-80mm I'd have to say that would probably cause you severe handling issues as once the car is lowered beyond 35-40mm, you'll start inducing positive camber when you go round bends due to the angle of the wishbones and suspension geometry. There are some great threads on here about suspension setups from people that have clearly tried every setup going. Just type suspension setups into the search bar and you'll be amazed at the amount of info there is. The one thing I've learned about the R's handling characteristics is that they are very sensitive, a small alteration of ride height or adding of strut braces can make a huge difference. Like I said use the search bar as its very helpfull when trying to find specific information
Good luck with everything
Ben
ive been told that th box definatley needs replacing if anyone got 1 going please please please phone me 07970669505 