I have an old G4 eMac And I have some older G4 and G3 iMac, So could I hook a Mac Mini up to them wiht out Networking them, because i want to use a screen on one for a display
Unfortunately you can't use an iMac or eMac as an external display for another machine, so you'd need another monitor. Good CRTs are cheap though, and well within the budget you mentioned. You can upgrade the RAM in a Mini w/o problem.