I would wait for / go find a used Series II with a dead BOSS or other controller and replace it with a PC based controller and the appropriate stepper / servo motor drives. If you want to see my retrofit story it's on this board. Pretty sure I'm at about $8K including the machine.
http://www.cnczone.com/forums/showthread.php?t=8749
Now you said you just want the big Z travel for the ability to change tools? Is that right? I think if you use tool holders for your drill bits you'll avoid the problem. Also, use shorter bits. Also, you'll probably end up doing a lot of "drilling" with center cutting end mills. I have had times when I've had to assemble the drill rig a piece at a time. First the tapered spindle, then the chuck, then the drill........ So yeah, it would be great to have 20 inches of travel and not lose reference.
You could put a DRO on the knee and hand crank it. Or fab up some kind of optical home finding "switch" or something. If the machine you get has a DRO you could move the DRO to the knee. OBTW, the knee on mine has a dial indicator. But I have not found it to be as reliable as I would like.