transmission upgrade? conversion?

[mr sweattypits]

hi all, i am about 140 miles and $1800 into mercedes ownership. i scored an 80 300 sd turbo, 116 body style. overall seems like a good car, usually. mine has been seriously neglected. is there something to swap in besides the 4speed slush box? it seems the 240d 4 sp manuals don't cut it on a 3L. What if i lower the diff ratio? the 5 speeds seem rare in the u.s. (anyone got one for sale??) will a 5-speed off a chevy s-10 fit-up or any other vehicles? I am looking for a die hard solid car, even if i have to build it myself, that is cheap to run. as far as conversions i am not afraid to chop, weld or butcher.
i am not spoiling a classic, this thing has had alot of problems surprise me like a brake pad worn past the sensor and destroy a caliper and disc and a supposed "rebuilt transmission" that looks that clean with clean fluid be toasted for instance. its only been 2 weeks i have had this car and it is seeming like i got trouble