First of all, did you make sure that the previous owner had replaced the evaporator. If not, I hope you bought the car dirt cheap.
1. Your first problem sounds like the transmission computer. Any MB indie should be able to fix this.
2. I bet Speedo assembly needs replacing. Why would the owner w/ all the expensive mods not want to fix a cable. He was probably told to change the very expensive unit.
3. Another common problem w/ W140. Do a search. But still an MB indie can do it.
I bet that engine is on its last breath with all that extra weight. You dont feel underpowered. I bet w/o the evap, you are looking at a $2500 job right now. But you hoped to be buried in it. W140 is not a diyer chasis like 300E W124. In fact, many mechanics dont understand its complex vaccum and electrical systems let alone a novice.