Grime = no problem. It's fun to clean grimy games, I think. Wear is a problem if you think it is. Will seriously reduce the resale value of the machine. Ball missing error is probably the trough optos being dirty and / or broken.
FTs seem to be selling around $1500-$1800 lately for shopped functional ones, I personally wouldn't go much north of $1000, but I don't want one at all. $1200 seems rather reasonable to me. But again, only if you don't mind the playfield.