just use legal ROMS that you have just to be in the safe side or the funds you collect might end up in court fees 
Play it safe....Doing good deeds does not mean its ok to do illegal activities.
Nope - the MAME license says you cannot charge people to play MAME. Doesn't matter whether the ROMS are legal or not, it would still be a violation of the license. For that matter, I think Starroms has a license for "personal home use" or sth, so you're probalby in trouble with them as well.
Three options -
If you put MAME on it, I doubt anyone is going to be able to check it out - maybe the tax collectors and regulators that keep up with regular arcade machines - you might want to check into that aspect.
Technically, I think you could run Retrocade or RAINE on it for pay and not violate the MAME license, although you still would probably be violating the copyrights on the ROMS, IMHO.
To be really legal, you would probably need a dedicated machine - i.e. arcade PCB, no computer, etc. Even then, there may be local licensing tax and other regulations.