Do an ebay search for a 420 SEL. There is one in Ontario selling tomorrow, with 143K on it.
Putting 15K$ into this car over the next couple of years is way too much to invest in a car that age.
I'd also do a compression check yourself to find out what you are up against. A gauge is only a few dollars and easy to use. Lots of ways to foul plugs other than bad compression. How does the car run? Does it have decent power? (with clean plugs)
By all means find an independent shop that will give you a good price. If you find a wrecking yard with a good engine, they will probably also install it for you or can recommend someone who will do it cheaply. If you can't find a local one, do an internet search for used engines.
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet
'86 560SL 124K Miles
'94 320E Wagon, 74K Miles