I'm a firm believer in "If you can't do the work yourself, buy new, because labor will kill any savings an older motor will afford you".
But new motors are way out of most folks reach these days. New larger motors are rare on the water these days, Its rare to even see anything four stroke. Even 25 year old motors are bringing at least half of what the same size new motor will bring. There are cheaper used motors but they're usually rough, having spent years in saltwater with no care. I find more newer motors though with bad salt damage than I do older one's.
I have a 1989 40hp motor on an open boat, I bought it for $400 from a guy selling off his dads boat stuff. The motor was fine running wise, but it needed a power tilt unit, the correct prop for my boat, and the controls that came with it wouldn't work on my boat. I ended up giving $240 for a good set of controls at a fisherman's fleamarket, the new prop was $75 online, and I installed a complete water pump kit, which was $90 at a local dealer (OEM). Plus another $90 or so in minor parts to hook it up, (fuel hose, fittings, spark plugs, a fuel filter kit, etc. So in the end I had around $900 in a motor for my boat. Its been great so far, but I did have one hitch in the deal.
One of the big wind storms we had put a tree down on my boat, that meant I needed a new hull. The new hull, took a long shaft motor, so I had to hunt down a 40hp parts motor with a good lower and mid to convert my motor over. The parts motor with a blown power head cost me $350 and a 200 mile drive. The gaskets, plus I had to reseal the used lower unit, all cost me round $210 or so by the time it was up and running as a long shaft motor. Now I have a two tone half Johnson, half Evinrude, (white and blue) combination motor with a few black Tracker parts mixed in as well.
But the alternative was spending twice as much for a turnkey motor, hoping it was really turn key after I got it home. This way I know what I have and can rely on it.
I was at a dealer last week that had an early 80's 40hp for sale, it looked really clean but the guy said it was just a super clean survivor. They had sheet of paper on it with a list of prices, Cash and carry, $1,995, They install it on your boat that already had an OMC with proper controls, $2,499, if they install it on a boat which needs a full OMC conversion, with new cables and controls, $3,599. I stopped by there today and it was gone, the guy said they installed it on a skiff on Friday.