I like to look at how clean/dirty the engine bay is for one, because it is often neglected. If the motor is clean and nice, like mine
, and not rusty and dirty, then chances are the car is a keeper.
Checking tyres for brand name and match is a good start too. For me, the first thing I look at is the overall condition and it is more important to me than the mechanical stuff (to a point, you can tell by driving the car, if the motor/trans has been maintained) as mechanical parts can be replaced, but bad paint and interior cannot be renewed as easily and shows a general lack of care by the owner. Plus more often than not, the cleanest original car is the best mechanically. If a car has been painted, I walk away, unless it has been documented why, because there is no reason for a more modern, post-85, Merc to have bad paint, IMHO. Just look at my car, the paint is darn good and is original, if the owner has respects his/her car, they will maintain the mechanicals as well as the interior/exterior. I've looked at a lot of used cars and its never one particular thing that gets me staying or walking, but the general condition. Has the leather been conditioned or is it torn, are the carpets clean and stain free, has the wood been polished, are the door panels dirty, etc. I guess I'm just the obsessive-compulsive owner (OCD)....
For example, this weekend over Turkey-day, I removed the whole interior of my w126 to clean the carpets, headliner, rear-shelf, etc… I reinstalled the driver’s seat to make the car usable, but there are no carpets underneath and the power adjustments and memory are unplugged at the moment.
Also look for factory original parts... clear corners are cool for the kids, but that means they spent there money on that and not on other things, like cruise control, air-conditioning, window regulators, etc. My own personal bias says that a car with mud-guards is a good one because it shows the previous owner cared more than most about rock-chips and scratches, but that's just me.:p
link to pics of the w126 on MBWorld thread