@PPT - To my knowledge the service of a 500 is far more than just a re-cap, bearing in mind there are 2 units involved. There were/are some pictures on the old Forum showing the strip down/re-build etc. Sadly, the search function on the old Forum is only very basic at this time (the searches only revealing thread titles it seems).
The DR’ing upgrade adds to this - mine also had a casework change to the rear (heatsink IIRC), done as part of the package.
Personally, I’d go for a 2000 model 500 and have it DR’ed and serviced (if required) at the same time on the basis the cost of acquisition reflects the absence of the service, should this be the case(?). If it hasn’t been serviced, then it’s highly likely to be ‘under the weather’ in performance terms.
If it has, then check out the service paperwork. Naim suggest 500 series kit service timelines are >10Y’s (perhaps up to 15).
Yes, the 500 can be used with a 252 - the signal cables to the 500 head unit are fed from the 252’s Supercap - these should come with the 500.
Attached is a link from the old Forum on DR-ing a 500.