I would agree 100% with what other people have said, being in a partnership could be tricky. I debated it before setting up myself, the only benefit I could think of was having a shoulder to cry on if it went tits up!!!
Be brave, get a proper business plan, go to the banks and borrow the money.
If you get busy enough for another mechanic, employ one and pay him £10 an hour, don't split your profits 50/50 with a partner
I had a few sleepless nights at the beginning, but it seems to have worked out for me. By the way, I dream of doing 45 MOT's a week. Easy money!!