Given the scant few machines, lack of features, and limited reach of the format (PAL only), no buying guide is needed. You'll essentially buy whatever is available. It's not like VHS VCRs, Hi8 cameras, etc, where you have choices, and features matter.
But also note this:
Originally Posted by Wikipedia
A key intention of the V2000 format, thanks to DTF, was tape compatibility. A tape from any machine should play perfectly on any other machine. Unfortunately, when the VR2020 reached the shops it was discovered that its audio head was 2.5mm out of position compared to that on Grundig's Video 2×4. This meant that the sound would be out of sync with the picture when played back on the other type of machine. Both manufacturers' production lines hastily moved the audio head 1.25mm to a common position, but compatibility issues remained for recordings made on the first generation of machines.
has more details on this issue.
Essentially, the V2020 decks are not what you want, if you have V2000 recordings. And probably vice versa.
To further what latreche said above:
highly prone to problems, and indeed a survey in the 1980's found that V2000 machines were significantly more likely to have needed repairs than Betamax or VHS decks.
And it being now the 2020s, the decks all are guaranteed broken, junk, parts, unless refurb'd by somebody with skills to do it.