I've had the SD1 (LT77) gearbox in my seight since i built it in 1997, although i brought a heavy duty type suffix 'D' i think that has better internals, upgraded my engine to a 4.6 engine, with somwhere close to 300bhp 2 years back.
The lt77 was originally fitted to the sd1 and many other heavy cars, even the vitesse (190bhp) version had one, but i think in a light westfield it does fine unless you are a 0-60 in as fast as possible type driver, it will last and give you good service, although there are better but expensive options.
Must say my lt77 isnt brilliant but does the job, although i would like to replace it with maybe the R380, but i am moving house, and this comes first.
At the end of the day, if it breaks you will have to fix it or replace it, this was my option when i fitted the 4.6 engine, i was prepared to break it, and when it did or does i will replace it with something better.
The T5 is excellent but will need chassis mods, and cost you close to £2500 unless you can find a used one for less, so is it really worth it.
Dont worry to much, with what people will tell you all kinds of horror stories about the 4.6 engine, some expensive if things go wrong, only exprience will help you understand what applies to you.
I you want to discuss anything further, i can let you have my number a proper chat.
hope this helps.
scott