SP DG seems to meet all your requirements as someone else previously suggested. If you can compromise on the machine being electronic, you might take a look at the ********* ML100 distributed by Alpha:
http://alphatennis.com/Machines2.htm
For a couple of frames/month, I would probably get a Alpha 4000 ($549 list) for myself.
http://alphatennis.com/Machines.htm
I recommended one for a former stringing customer of mine that wanted to do 15-20 frames/year and he has been very happy with it.