I agree these machines are "worth" it since you usually pick them up for next to nothing to begin with - I've gotten them for as little as $20 - and the $75 you spend on the wiring gives you a very fine machine for around $100. You can't beat that with ANY new machine at that price.
But anyone considering a potted motor machine like a 15-91 or 201 should just be aware that the wiring usually needs some attention and it is possible to DIY if you have some basic skills in soldering and crimping wires. I've done it a couple of times, and honestly don't ever want to do it again, but at least I know it CAN be done!