What brm said ! Also invest in a compression gauge. A compression check from one cylinder to the next can tell you the health of the motor. When you take the spark plugs out run your finger along the treads. Have seen metal shavings show up on my finger that can't be seen on the plug. Also add the cost of a new impellar for the water pump unless they can show you a reciept in the last year or two.
Good luck !

