Ra RA's have the W series gearboxes which are stronger than the t50's plus they seem to be more user friendly for conversions to the likes of 3tgte's and 1ggte's
your pretty much stuck with the t50 unless you want to spend big biccies on a sprinter W series coversion ..
you can get decent ra2X's with 18rg's and w50 boxes for under 5k these days - they would be the best starting point for modified carby old School toyota - just takes a bit of looking to find them.
OR on a slightly lesser scale - theres alot of ta22's with 2tg's in them that can be picked up for under 4K ..
IMHO sprinters are only good as a fun car if they have the 4age or 4agze in them - but the 4ag-ZE will kill t50 gearboxes in quick order if you start making them go harder.
At least with the Old 2t/3t motor in the old school celica - you can make them go hard - with camming , webers and tuning , the 4ac leaves a lot to be desired ..