I believe it is 17 PTO horsepower. Most of the Japanese tractors of that vintage (except Kubota) had their model numbers that designated PTO horsepower first, then other modifiers in the rest of the digits like transmission differences, four wheel drive vs two wheel drive, etc.
I would buy a 4 foot cutter for a 17 horsepower tractor. If your field is lightly overgrown, a 5 foot cutter may be usable, but especially with hilly terrain I would rather opt for the conservative end of things.