1988 Other BMW Models My Car will not go forward

1988 Other BMW Models 6 cyl Automatic

My car will not go forward in anyone of the gears. I got this car from my uncle the car was setting in the yard for a year. He took it to a mechanic friend of his who inspected it and it past. Well I did ran over a crub I think it put a hole in the pipe of the transmission but it was fix by him. At the time the fuild did almost empty out but he did put more in than it started to idle high then when it was in the first gear it would take some effort to move then I had to put it in second just to drive then I had to put it into third to drive then it would not go forward but it did go in reverse. Now my uncle says I have to replace the whole transmission and I was wondering if I had to bcuz some friends of mine says it just the seals but now the battery is acting up but its brand new and I only had the car for about a month now. I did have a amp and stereo installed before this happen but after the hole in the pipe so I don't know what is the problem and what to do to fix this car and my uncle is now as we speak trying to find a new transmission are a used one but he is saying I have to pay between $240 or $400 to get the part and $400 for him to fix it so I just need to know what do I do. Thank You oh yeah the car is a 1988 BMW 528E. Thank You again.