Hey! (Automatic Transmission)

I recently purchased a vehicle from a private seller! It drove fine for about about 4/5 days until one day it would accelerate when I hit the gas! So I would turn the car off for about 2 minutes and turn it back on than it accelerated for a min. Started to do it again and when I press the brakes I feel an aggressive vibration. I had it looked at by two mechanics. One said I needed a transmission filter, fluid, and a tuneup. Another mechanic said the second and third gear are "Completely Gone" and I will have to replace the transmission. What should I do? Can I just get them to rebuild it or just replace it? Or should I get the filter and transmission fluid along with the tuneup??