Buying a lawn tractor or ride on mower for people in Western Europe. Due to the fact that our summers are invariably showery, the first requirement is POWER. The best power will come from a diesel engine.(Because they do not die under load) Hydro units are easier to operate than machines with clutch & gears. This adds huge cost to a new or used machine,(about 25%) but to be able to control the forward speed, by pressure on a foot pedal is worth it, don't forget that the damper the grass, the slower you have to go. The next requirement is a straight through feed for the cuttings, if you have the need to collect. (If not go for a mulcher). Match the deck size to the engine power. About half of one horse power per deck width inch. The width of the cutting deck is very important also, and it is important to buy a machine which will fit your situation. The final thought should be the difference between the deck or cutting width, and the tractor or wheel width, as it is important to be able to cut to the edges when going round outside curves. The deck should be wider at one side than the other to facilitate curve follow & width restriction.