It may be time for a transmission, however if the only problem is 3-4 shift and it has not been doing it too long, change the 3-4 accumulator spring to stiffer one and it will take care of 3-4 flare.
Note - if trans really bad, may only be a temp fix.
Never, never go for a used unless you know exactly what car it came out of and had the oppertunity to drive prior.
Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture