Hello house

I recently acquired a V40(1996 16 valve engine) from someone and I've been facing some issues with it

Firstly, the RPM just shoots up by itself and revs so high when the engine is started, I took it to a mechanic who said it's electrical issues and he did whatever he did and that stopped

Now the car is fine when in P or N, but when engaged to D it stalls and engine dies off, and when the engine manages to stay on, acceleration is very poor(like a horse trying to pull a skyscraper along), however, when put to reverse gear R, the car runs smooth and fine

I'm really scared of letting the trial by error mechanics we have over here(Nigeria) permanently ruin something for me, so I'll appreciate if anyone who has had similar issues will help me out with a solution

NB: Vehicle has been parked for over 2years prior to my possessing it now.

Thanks