By 1993(or so) Mercury was in charge of production. By 1999 they were at their best. Check that it hasn't EVER been overheated. The rear of the block gets hottest first. Look for evidence of discolored/chalky paint. Nice clean shiny paint on the powerhead means no overheat and no white powdery residue means no salt. Beware ANY re-painting. Look for oily residue around/under ignition plate(bad crank seal). Look for oily residue on the block parting line(air leak). Check that the engine idles nice and low with out popping/coughing/spitting. There's only one carb with one idle screw. If it won't adjust out nice and smooth, then, there is a bad crank seal or parting-line air leak(or other air leak). Look for milky oil in lower unit; have the dealer crack the lower fill screw and examine the first couple drops of lube that come out. He'll know the drill. Of course, he may have just changed the lube.
These engines evolved from WestBend in the 1950s. Chrysler bought West Bend in 1965-6. They used all high-tech space-age materials. They were light simple, powerful, and long lasting. I grew up with a 35hp Chrysler on my Dads boat. U.S. Marine bought the division in 1984-5, made them as cheap as possible with inferior materials and workmanship. Mercury inherited all the Chrysler tooling when they bought out US Marine. Took a while to get them squared away again, but they were gone forever by 2000. They got their bad rep from US Marine, purveyors of shiny junk.
If you find a nice well cared for 40 Force, it could serve you well for a very long time. If you find a junky one stay away. In either case you should be able to get a real bargain because of their bad rep.
