I have a 73 140 HP Mercuriser and it's an easy engine to work on, basic Chevy 4 cylinder engine. Get a Seloc manual for the engine as it gives you all the details you'll need to work on the engine and outdrive.
At a bare minimum change the fuel filter, plugs, oil and oil filter, and if they look old the spark plug wires, cap, rotor and points if applicable. Check the belt(s) and hoses. Check the exhaust manifold for cracking and water leaks. Inspect the belows (rubber boots between the outdrive and gimble housing on outside of boat) for holes, cracking and getting squishy-mushy, if they are leaking, water can get into the boat and the gimble bearing can go bad from water intrusion. The outdrive lube will need to be drained and inspected for metal wear and water contamination, then refill lube. If the fluid looks milky or water comes out with the lube you need to find the leak and fix it. The outdrive should be removed (you will need a gasket kit for this), check the gimble bearing and engine coupler alignment (take to a marine shop for alignment if you don't have the tool), grease the coupler splines with "spline grease" and grease the U-joints.
About the only thing I found to be a real PITA was adjusting the shifter/outdrive cables. Following the adjustment procedure in the manual got it close, but there was 30 years of wear and tear on the cables and pivot points so I had to tweek the adjustment a bit to get reverse to engage properly.
Your boat is close to 30 years old and you don't know how much abuse and neglect it has suffered, so it's best to check things and do some preventative maintanance before you get stuck on the water with something stupid like a clogged fuel filter.
I've had the 73 for about 10 years and here's a list of things that have gone wrong or were repaired:
- Outdrive was rebuilt before I got the boat
- outdrive wouldn't go into reverse properly, adjust shifter cables
- engine rebuilt including new starter, alternator, water pump
- carb rebuild
- replaced cracked exhaust manifold
- exhaust flapper valve was burned out causing the engine to flood with water when the engine died, replaced flapper valve
- replaced ignition switch which would cause the engine to die
- Bent water pump pulley was replaced, $20 from a chevy dealer
- fuel pump replaced (though it probably didn't need to be replaced)
- fuel line from tank to fuel pump replaced, it was cracked at the fuel tank fitting and causing the fuel pump to suck air during start-up
- installed fuel filters before fuel pump and before carb because of rusty gas tank
- distributor bushings bad causing rough idle, replaced distributor
- outdrive output shaft seals leaking, replaced seals
- bellows were soft, cracked and leaking, replace belows, reseal outdrive, replace gimble bearing, U-joints, align gimble bearing and engine coupler
- Engine front motor mount bolts stripped out and loose, fiberglassed in new support for front motor mount and align gimble bearing and engine coupler.
- Buddy found a sand bar in the river, repair prop :104167739
- Buddy hit a log in river, repair prop :104167739
- I found the gravel lake bottom, repair prop
- I forgot to raise the outdrive when pulling the boat out of the water, repair prop
- new props on clearance sale at Iboats for $60, bought 3 of them
- Plus, a bunch of other things I have probably forgotten
Hope this helps to some degree