I`ve spoken briefly with the staff at Watchfinder in Leeds. I got the impression that all watches receive cosmetic attention to get them looking good, but they don`t necessarily receive a full service. This poses something of a dilemma for buyers; even if the watch is giving good amplitude and performing well on a timegrapher it's not possible to predict how long it'll contiue to do so. On the one hand, it makes no sense to service a watch if it doesn`t need it, but on the other hand it is in the buyer's interests to know his watch will run for a few years without needing expenditure.
I don`t know whether Watchfinder show proof of recent service if it exists with a watch; it would help the buyer if they did but it would also give a basis for charging a higher price! Personally I think Watchfinder's prices are too high in many cases, particularly for watches that are 10+ years old.
As for the OP's concerns about seals and gaskets, I find that they don`t deteriorate significantly on modern watches. I`ve taken seals out of Omega SMPs that are over 10 years old and they are still serviceable. The modern synthetic rubbers don`t seem to deteriorate and in many case they get replaced as a mater of course at service time, they don`t necessarily get replaced because they're deteriorating. Provided the watch will pass a 6bar pressure test (which it should easily do) I don`t think the OP should be concerned, I certainly wouldn`t be worrying about deteriorating seals on a modern Explorer but I`d be concerned at having to fork out £400 to get the watch serviced in a the short term.
My advice is to play hardball on the price if there's no evidence of recent service by Rolex or a Rolex accredited independent. In an ideal world Watchfinder would show the timegrapher data for each watch and confirm that it's running well.......dealers just don`t want to do this for some reason and they prefer some vague statement delivered in reassuring tones.
I share the OP's enthusiasm for the 36mm Explorer....mine's on my wrist at the moment. They are the most rugged watch in the Rolex range in my view because each surface can be refinished fairly easily. No fancy bezel insert or fluted edges to get damaged, and that's a big plus in my eyes. However, given the high prices being asked by such as Watchfinder a new one starts to make more sense.....if you can live with the bigger size which personally I wouldn`t want. That's another dilemma thesedays, finding nice watches that are only 36-38mm in size.
Back to the garage for me, to resume battle with the MG. I`d rather be fixing watches than working on this thing, the novelty's well and truly worn off, sick of the smell of paint, oil and white spirit
...........needless to say the Explorer will be coming off and getting put somewhere safe!
Paul