Hi and another welcome to the forum .
I'm guessing your car will be a late W124 ( squared off body shape with rectangular headlamps ) or could it just be an early W210 ( more rounded body shape and four round headlamps ) ?
Either way , you should have a safe , reliable car which should offer reasonable fuel economy . Both models are solidly built with good crash protection for the little ones in the back should the worst happen .
Mechanically , the two models will be quite similar with little to go wrong - just check the basics over to be safe - has the car been serviced recently ? If you look after them simply by changing fluids , filters , brake pads at the proper intervals , not much else will go wrong mechanically . Just check that the tyres are in good condition , brakes are working properly with discs/pads in good condition and exhaust system sound as these items could cost the price of your car to put right and , while none of these is anything other than routine maintenance , are often the sorts of things needing done to a 'bargain' car and why the previous owner has decided to sell rather than spend the money on the car .
You did say the car has a long MOT ? If so there ought not to be problems underneath with springs , brake pipes or suspension bushes - all of which can go on these cars after so many years - I'm not trying to be a prophet of doom here , but just trying to list things I know from experience can go on these cars so you don't get your fingers burnt !
Both W124 and W210 models can suffer from rust - W124's tend to go on the front wings before anywhere else - they are easy to replace as they just bolt on - also check the inner wings just behind the headlamps , also the areas near the rear jacking points and rear suspension mounts .
Some W210 models had corrossion problems due to poor paintwork and rusted just about everywhere - most of these cars have now been scrapped and only the better ones tend to be left - with one of these still check front wings , edges of door and boot panels , front spring perches .
On all models compare sitting in the drivers seat to the passenger one ! Often springs break in the base of the drivers seat causing it to sag and be uncomfortable - you might even find a broken piece of spring poking up cheekily through the fabric to attack your nether regions ! Seats can easily and cheaply be rebuilt using the base from a passenger seat out of a scrapyard , so don't worry .
You would be very unlucky indeed to find more than one or two of the above problems in any single car , but they are worth checking for before you buy .
Good luck , and let's have some pictures of the car if you get it .
