I agree, do not install a mouse on the control panel, simply take the mouse out from a hidden keyboard drawer or coin door and use it when needed. You should just wait for "menace" to sell his used (but refurbished) trackballs. I think I remember him selling them for $30 and people who have bought them like them. Anyway, I think a trackball is a must. Even if you do not play trackball games, someone is bound to come over and ask to play Golden Tee, Centipede, Missile Command, etc.. on your cabinet.
I would go as far to say that if you think spending an extra $30 (or even $70) for a new trackball is too much money at the moment, then you might want to wait a bit before building a new arcade cabinet from scratch since you might need the money for other things. If you plan on keeping the cabinet for a very long time, then money probably should not be an issue since over time it really is not a lot of money at all. Please don't take these statements personally .... I was just giving my opinion since you stated the trackball was too expensive to add right now. Of course if you just do not want a trackball since you do not like them, then this is a different story, but since you mentioned you were not getting one since it is too expensive, then I think you are making a mistake. You just do not want someone coming over to see your brand new cabinet and the first thing they say is .... lets play some "Golden Tee" ..... it will make you feel ill