FWIW-I have owened a 5 sp Cat and 6 sp, and driven a member's Ultralight w/Honda gear shift. I liked the Honda best (short, precise shifts), the 5 sp Cat next, similar just not quite as slick, and 6 sp least (very hard to tell what gear you are in or to find the next, exp after all that "muscle memory" associated w/5 sp tells you you are in the wrong gear). As I am learning it, tho, it is growing on me. It is nice to have 2 more usable gears than the 5 sp , in wh/both 1st and 5th are irrelvant at most tracks. But all these trannys are excellent.
Skip Cannon (see scannon post above) has a 300 hp Miata turbo in his Caterham. Its cheap, relaible and tons of parts are available. That wld be on my short list of engine options.The Miata 5sp seems to be robust and they make a 6 sp as well. I owned a Miata 5 sp and wld rank its transmission equal to or better than the 5 sp T9 transmission. I suspect Skip had a build diary that shows how he put his car together.