Buying an NAD3020
The first 3020 models appeared on the market in 1979 and were designed in the UK but assembled by Fulet Electronics in Taiwan to keep costs well under the established competition. The performance was superb and really did cause a stir amongst the higher priced rivals! Current (2021) used prices for this model range from £100 to £250 with the latter price applying to mint boxed examples with no faults. The very first models identifiable with the additional small PCB for the regulated supplies mounted vertically at the rear didn't have an audio power on mute so a bit of a 'thump' is normal on first switch on as the dc conditions settle. Later versions had such a circuit fitted giving a 5 second audio mute thus avoiding such noises (until it fails which is very common in which case you will get no output whatsoever!)
Common faults are intermittent noisy controls, hum on output and arcing power switches. The flimsy plastic fascia is prone to losing corners as it's now quite brittle and few sellers ever package properly for shipping. However do not let the budget appearance put you off as when overhauled and working as designed they really are superb sounding amplifiers.
See here for a recent 'What Hifi' review of an original example.