I wouldn't touch any of them. Here in the States we have only two or three places able to restore Naks professionally. Tandberg or Revox - you have to DIY and pray that heads and motors are still solid To get all the necessary gizmos and calibration tapes (back-tension, mirror cassette do not come for sale often!) will cost you more than professionally calibrated ZX-7 But I agree with @Mister X - if I ever see one of these beauties in real life, I might change my opinion in split-second (as I often do!)
There's a website somewhere where the guy builds a mirrored cassette but yes it stinks that the people that can calibrate these old units are non-existant.
in the netherlands i know a revox/studer repairshop. so, the studer a710 isn't a problem to fix. only prices of the 710 went sky high. oja, motors of revox/studer are unbreakable. iow they are very very good. the heads are made by sony, perhaps studer modified the tandberg is more difficult. i could buy 910, 15 years ago +/-1250 euro, founding too expensive. but this one was always my number one. in the end bought a revox 215 for 150 euro, better deal.
