Welcome, Soren, to the world of 113's.
There is an excellent book by Frank Barrett (I've attached a picture of the front cover; it is available directly from Toad Hall Publishers
http://www.toadhallbook.com/ or from Amazon.com)that gives a lot of explicit detail on the vehicle and what to look for when evaluating. I found the book valuable; it was only $18.95. You should buy it and read the section on the 113's prior to purchase. My basket-case car came from my Uncle, but after I got it I bought Barrett's book as well as quite a few others (Toad Hall carries most of them) out there (just do an Amazon search) so I was armed with a lot of published information when I began my restoration. I bought a similar book to Barrett's years ago regarding Triumph Stag cars, and wisely learned to STEER CLEAR of Stags!
In addition to this group, which spun off of the Yahoo group (not for any political reasons it was just that the prepackaged Yahoo format was limiting), there is the Yahoo group
http://autos.groups.yahoo.com/group/W113/ as well as another group out there
http://pagoda113.com/ that cater specifically to this vehicle. No matter what problem you have or part you need or advice, you are amongst people who have been there and done that before, and we have quite a few experts among us--people who have had their cars for 30 years or more like Richard Madison; people like Joe Alexander who knows them mechanically inside and out; Doug Kim who is a treasure trove of detailed information--and that's just three people! Others have show cars; some have raced; others are daily drivers. We all share information to keep the cars running that's why we've organized, as it were.
Richard is correct in that the "Kinder seat" was a rare option, but who today would actually put an untethered child in that seat? It has far more value as an historical oddity or option then a practical seat in which to tool around with your kid. I think you can tell if your seat was factory original by the data card. Other dealer installed items were Blaupunkt (Only Beckers came from the factory) radios and air conditioning.
You might need to tell people where you live, and you might find someone willing to help you out or point you to a local mechanic who can evaluate the car for you. I'd have to agree with Richard about the price. If the car runs well, and the body is not too bad, the price seems fair.
If you are totally new to the world of classic Mercedes and SL's you'll be pleased to know that nearly every part you might need is available, many of them quite reasonable in price. Some are available as aftermarket, others as OEM only. There are some items such as the hood, radiator, soft top frame and others that are extremely expensive, but most other items are as reasonable as parts get. Sadly the bumper you might want isn't in the cheap category, expect to pay about $500 each (there are two of them)for the rear set. eBay is a place to look for them, also the STAR classifieds.
If you are going to buy the car, I suggest joining the MBCA and thus subscribing to the Star.
Good luck!!
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Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored