I'm assuming that you trust your friend and he'll give honest answers when you ask the following questions. In general if you're buying it from someone you don't necessarily trust, you'll probably want to inspect it and let your eyes tell you if they agree with the answers. (See below.)
- Was the oil changed annually?
- Spark plug?
- Was gas drained from the tank at the end of the season? (It should be drained before storage -- or run until it is out of gas.)
- Was it stored indoors?
A "no" to any of the above takes some life off the engine.
Give the machine a visual inspection:
- Are all the parts present? (Whichever are applicable: grass catcher / bag, deflectors, guards, flaps.)
- Where there are bolts holding pieces of the machine (e.g. the push handle), are they tight? Are any missing? (My mower is missing a couple of the original plastic knobs here -- you'll know when you inspect it.)
- Is anything held on with duct tape? (Don't laugh -- I speak from experience!)
- Is there any rust? Severe rust on the frame/chassis would be a deal breaker for me. A little rust on the muffler isn't the end of the world.
- Check the dipstick -- does it have enough oil? If it doesn't, you might question (a) has it been well maintained (i.e. does your friend often run it low on oil) and/or (b) does it burn oil? For (b) ask your friend if he has to refill the oil regularly.
- Is the blade sharp? If the blade is in rough shape -- dull and/or with big dings, you may suspect that the machine in general hasn't been well maintained.
- Look overall for wear and good running:
- wheels; are they heavily worn? This would point to a lot of use.
- chassis / frame -- are there dings, scratches, etc. This would point to a lot of rough use.
- starter -- is the rope and handle intact, does the rope pull freely, does it start on the first pull? An engine that won't start right up has problems that you may not want to get involved with.
- engine -- does it smoke when it first starts or during running? Again, this could point to problems.
My experience with an old machine -- a 1970s-era rototiller -- is that replacement parts have been easy, but I've only replaced belts, spark plugs, and the pull start rope.