I have been running the 270 kit for many years now. No issues with the gearbox so long as you use the clutch for the changes and don't treat it like a 250 two stroke motocrosser.
There is quite a thread on this under "KLX250 a shared disease"
I used a 72mm MTC piston from Florida. Not too expensive either. A rebore here in the UK is about £35-£40. The taking apart and putting back together is a doddle. You don't even have to take the engine out to do it. I also built up a head with Megacycle cam and 1"1/2 exhaust. It is still gutless, but has the grunt of an XT500 !
If you are being quoted those sort of prices I would go elsewhere !
You will find plenty of help on here for the KLX. They may be gutless (it will improve a bit), but if treated right the are very reliable and handle like a dream.