I’ve bought and sold A4 and AR MK II and both felt like new. I think both are sold pretty quickly after only some fiddling, judging from the condition and very few aves patterns/kits of the units I’ve bought.
With AR you might look a bit closer and ask some questions, as early versions apparently had some pad issue that users might not have fixed after buying them. Shouldn’t be an issue with black units I guess.
I never saw MKIs in person but much prefer everything about MK II units, except that MKI are a lot smaller and thus better to use in combination. AR and A4 MK II are wonderful desktop synths by themselves and look gorgeous, but take up a lot of space next to each other. Also, if you have an OT MK II next to them, it looks a bit off. Might be better with all black versions though.