It appears that you have a Craftsman garden tractor. These units come with either a Kohler or Onan twin cylinder engine that when serviced yearly will last many many years. I sold one with a Kohler twin this summer for $600.00. They are good old machines and although most mower parts are expensive, parts for this unit are relatively cheap.
Things to check for:
1. Pull the dipstick and look at the level qnd color of the oil. If it is really black....not a good sign.
Only an idiot would sell a mower with nasty oil in it. And if they do, it has been run with the
dirty oil obviously. As long as there is oil on the stick it's not that big of an issue.
2. If the is no oil on the dipstick......walk away. It may run ok but leaving one low on oil is an old
trick to keep it from smoking. You get it home and top it off with oil, and find out the rings are
bad and it smokes like a cropduster.
3. Most everything else can be repaired or replaced at a reasonable cost. Keep your oil and air
filter clean and it will probably last many more years.. Be sure to test mow with it and check
all the transmission gears. If its a hydro...make sure and push the lever forward and back
quickly from neutral and make sure it has a good snap on take off.
Send me your mower model number and I can give you more info.