There are lots of threads on this, but they may take time to find, and @anon33182107 's summary looks pretty good to me.
In Olive, the in-demand boxes second-hand are probably the 72, 82 and 52 pre-amps, plus 140 and 250 (or 2 X 135 mono) power amp, plus Hicap or Supercap power supplies. I have owned all except 135s over the years and would still recommend all those on sound per £.
I’d personally rate a 72/Hicap/140 combo as better than modern Naim amplification up to about Supernait 3 level, and am happy to own an olive 52/ Supercap/ 250 - a bit more music and a bit less hi-fi precision, but very similar in enjoyment to a more recent 252/ Supercap/ 250 combo.
52s are hard to find and not getting cheaper. However an 82/Hicap/250 combo is broadly comparable to a more modern 282/ Hicap/ 250 combo and costs vastly less because 82s are cheap and plentiful.
In all those cases, the family differences will be evident. At the margin, and at given quality level, olive amplification is generally felt to be at least as musical and involving as recent Classic boxes, but to have marginally less grip on bass and marginally less fine detail and tonal neutrality. All those differences get a bit bigger when the modern kit in question has the DR upgrade.
For now at least, Naim no longer offers DR upgrades to old boxes and olive boxes can’t have it anyway. However, amps and power supplies with lots of capacitors do wear, and old amps sound duller and then increasingly unpleasant if they really need a service. Timing depends on use, but most of us reckon that anyone buying power amps and PSs in particular that have not been serviced in over a decade should probably budget on getting it done sooner rather than later.
Modern ‘Classic’ kit that is not DR-ed is increasingly cheaper than DR-ed kit, but those who particularly want more neutrality and detail often reckon the extra is worth it.
Before olive there were the chrome bumper models, and their pros and cons were IIRC just like olive, but the cons were a bit more noticeable - more boogie than neutrality, not ultra-quiet in background, not as good as peers at precise stereo images, and more unforgiving of bad sources or recordings.
Personally, the only time I would ignore older options is when the new options have demonstrably new tech, which really means streamers. For convenience, flexibility and sound quality, I avoided old streamers and went straight to NDX2, but even there some will be happy with the extreme value available on older kit (as long as you don’t want Qobuz, for example).
If you like the ‘Naim sound’ from modern kit, I’d be surprised if you don’t like olive kit or modern but pre-DR kit too. However, I wouldn’t encourage you to buy olive (or chrome bumper) if you have never heard any at all - the whole family difference may not suit you. Of course, if you are ok with selling on eBay, you can probably sell any mistakes for little or no net loss beyond some postage, but it is a bit of a faff if you don’t do it often.
This is a Naim-operated site, and we should not discuss here any cables that could be regarded as Naim-to-Naim, as Naim regards using them as ‘unauthorised modifications’. If people do insist on commenting here (a mistake I made myself) it just forces Richard Dane to spend time removing it.