I own a 2001 R170, basically the same car you're looking at. And I've been in the market for a clean 1997-2000 W202, so I know what you're going through.
First, I'd get another Mercedes shop/mechanic to do an inspection on the car. Have him pull codes, do a test drive, etc. But if you're buying from a dealer, maybe you can wangle some kind of guarantee as part of the deal, something like if you find anything material within 30 days, you get to take it back or get a discount.
As to the price, it seems a couple G's high to me. The car looks nice, and dealers certainly get more than private parties. But if you go look at the car, like it, I'd maybe open with a $5,000 bid and be a little unreasonable.
As for fuel mileage, I have the 6 speed, and am a total lead-foot. But I typically get about 19 MPG around town (sad, I know). And on the freeway I can get 27-30. I drove mine from Boston to San Francisco in December, and I averaged 27.6 MPG. That's probably not MUCH better than what you're driving now, especially around town. And the car you're buying will probably do a bit worse due to being an automatic.
Finally, be sure to check that the seats move. The relay for controlling the electric seats is famously erratic. If you test drive the car, try the seats before and after bumps. The part is fairly cheap (< $40), but it's a complete pain to access.
Also check the self-dimming rear-view mirror. If there are any bubbles, lines, etc, it will have to be replaced for about $500-600. (That's the figure I'd use in negotiation; they can actually be repaired for about $120.) And I believe this is more of an age factor than a mileage factor.
As for leaks, someone here wrote about all the drainage tubes in this car. I've got a couple that are clogged, but the main symptom is that the driver's seat belt gets damp if it's left in the rain. I don't think I'd notice anything on a test drive. See post here
Oh, the car is prone to rust inside the front fender where it joins the front bumper. you'd have to pull off the interior (plastic) fender guard to see it, but it might be worth it. That said, your car hasn't lived in the salt/snow belt, and IMHO, DC hardly counts. (Said a Bostonian.)
Anyway, Good Luck! And let us know what you do.
P.S. 61,000 miles isn't a big deal. There are plenty out there running strong with 3x that. Mine has 75-ish, and it's strong. I did recently need to replace the steering damper (you can find the post I did on it), and opted to replace the front shocks. (Probably unnecessary) My only problems are that seat relay and the rear view mirror. Oh, and last year I replaced the trunk lid supports, but that cost me something like $40 with parts from Amazon. (Which is a GREAT source of parts, BTW.)