Good advice, everyone. I've been watching for a Reece buttonhole machine and found one for around $300. These are amazing machines -- watching them do their thing is a beautiful sight. And I'm thinking a Kenmore zig-zag with all the bells and whistles (like the 18023) plus a free arm machine (Phoenix or Kenmore convertible, not sure) will be great.
I have two 201-2 machines, both in beautiful shape and I bought both of them for such low prices. I hate to part with them, so one might be my straight-stitch. Although the Pfaff 30 is a strong contender. (1.4 amps on a domestic straight stitch can kick some serious tail.) I think the 15-90 and 91 I have are too underpowered.
As for coverstitching and overlocking, I may just hang it all and get a Juki industrial that can do it all -- except keep my domestic 4-thread Bernina/Juki (MO-734DE) for hemming:
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OK, so I just purchased an industrial zig-zag machine about an hour ago. It's an Adler, very worn in but not worn out. I plugged it in, pressed the pedal, and experienced a little bit of heaven. I bought it on the spot. It does single and double needle sewing and has a clutch motor rather than a Servo. That's OK. I can upgrade later if I want.
It comes in an industrial wood topped table (not laminate or other garbage) on a metal frame with a full-function pedal that is throttle, disengages the presser foot lift, and stops on a dime. I love the hand wheel assembly with two wheels controlling the belt feed to the motor drive.
It needs oiling, cleaning, has a little bit of rust here and there but the price was just too good to pass up.