As seen on the Wayback Machine
http://web.archive.org/web/199807020...com/specs.html
the dry weight of a 1997 2518 was 2190 lbs, with a load capacity of 1169 lbs. Total max weight therefore 3359 lbs. This is a it less than today's smaller TMs, probably due to the smaller tires. Your trailer hasn't grown any, but your load may have, so a move to 14 inch tires is probably sensible. There is probably no need to go to 15 inch tires.
Bill