1996 BMW 328 Transmission slipping

Background: At about 145,000 milles, lost traction control, warning lights came on along with Check Engine light.
Shortly after, transmission started eratic shifting, but worked OK in manual mode. Took car to mechanic. Changed out a module, shifting in Drive mode now better.
149000 miles, transmission started acting up, slipping a little and tranmission light came on, while going into only 3rd and 4th gears as manual said it would. Some days were better than others as far as acting up.
151,000 miles, loosing 2nd and 3rd gears, slipping while starting forward motion, then catching on.
4th gear and Overdrive work fine as long as Trans> warning light stay off, and that can be reset by shutting down and restarting.
My question more or less concerns replacing transmission, should I throw good money after bad for a "used" transmission? I have chanhed clutches and transmissions in domestic vehicles, (Fords and Chevys).
Would like this vehicle to make it 50K more miles before replacing.
Should I shy away from used? Is rebuilt the way to go?
Does the traction control problem stem from the transmission? Will changing the trans out solve both problems?
I can handle an honest answer, even if it consists of me pushing the car over a cliff.

Thanks!