The PDF has prices for genuine Mercedes engines from MBNA through your local dealer, Rusty, Phil, etc. The prices are several months old though and could be higher now. And the dealers need to check stock in Germany to see if they are indeed available! I agree with you - the W140 is a fantastic car, and if you've owned it since new and would like to keep it another 5-10 years, it's worth fixing/replacing the engine.
As to what I would do? Hmmmm. Good question. I think I would attempt a compression test now, just for grins. Then keep driving the car until symptoms get significantly worse - which could be a LONG time. At that point (low power, bad smoking, etc), I'd do another compression test just for grins (and to see the difference from the original test), and just replace it with a crate motor (assuming one is available). If a long block can't be sourced at a reasonable price, a factory short block would be the next choice, re-using your existing head (with a freshen-up valve job, seals, etc). Last choice (IMO) would be an aftermarket rebuild from Metric Motors - not because they don't do good work (they do), but because the last time I checked, they're actually more expensive than the MB crate motors!
