Well... I think a service will run $200 - $400 depending on who you get to service; price may go up if replacement parts are needed (though if it's just running a consistent 15 secs slow, I bet it just needs an overhaul, fresh oils and regulation).
I have noticed some of these matte dial 16800s starting to creep up in price, but the usual median seems to be (realistically) right around $4800 or so in my opinion. I see several listed for $6k - $8k, but the fact that they've been on the market for as long as they have is evidence to me that they're priced too high.
I think the better piece for collecting purposes would be the 16660 matte, but those are definitely at a higher buy in.
Speaking on this piece specifically... That's a tough call. The dial is quite nice. Definitely good shape and usually the first thing to take into account when considering a vintage Rolex. However, the case is also an important aspect; given the condition of this one, it's a bit rough.
Personally, I'd say wait and be patient... 16800s are still out there, and usually for not much more than the one being offered in your post. It's always a tough call when it comes to this kind of stuff, because you might be waiting a long time before you find the piece you're really after in terms of condition, price, etc. Or, it could be tomorrow. But if there's any doubt... I'd say pass.