Welcome to the MB fold! The 450 SEL is not a particularly valuable car these days, and excellent cars can be had for modest prices. I saw one on eBay today---less than 1 day on the auction, and no bids at $800 starting bid.
With the 450 SEL, be prepared for high fuel consumption. These were early emissions days, and the 450 (US-spec) was a low-compression engine which consumed lots of fuel. Maintenance needs also may be high. The key to affordable ownership of one of these old cars is (1) buy a pristine, low-mileage example, (2) find an honest, low-priced independent repair shop, and (3) perform lots of the maintenance and repair yourself-----and preferably all three.
Look at cars with the Euro bumpers, and compare these to the US bumpers. Find out which you prefer. Speaking for myself, I greatly prefer the Euro car. An eBay Euro-bumper example with the 4.5-liter engine (VIN starts 116.033):
A US-spec bumper example with 6.9 liter engine (VIN starts with 116.036 instead of 033 for the 4.5-liter engine):
There also are 450SE (short wheelbase cars) as well. They were imported as USA models in 1973 and possibly 1-2 years after that; all later 450SE cars are Euro-spec. These are supposed to be the best-handling and most sporty of the 116 cars.
Still another consideration is a 350SE. The 350 was a short-stroke, high-compression, high-revving variant using the same 450 engine block, found in Euro-spec cars only, with about the same horsepower as the USA-spec 450 engine. The 350SE is rarely seen in the US as it was a "Gray Market" car only.
So there you have it.....drive 2 or 3 cars or more, if you can, as they will have distinct personalities. Some cars feel "old" while others seem much newer, transmissions shift very differently, etc. Be bold, and demanding, when it comes to price. If you buy a nice car, even for a modest price, you can still expect to spend thousands more on it, over several years of ownership. My first two Mercedes (280SEL 4.5 and 300SEL 4.5) did just that, yet I'm still a Mercedes enthusiast, 20+ years later.
Best of luck to you------