Specify and detail.
Without limits...
A single character of any level will win. He spends the gold to hire an army of whatever level of knights will suffice for any one individual to attack and do damage. Send them en masse.
A single character of any level w/ fire immunity, and enhanced DR from magic items to prevent physical damages. Down goes dragon sooner or later.
A Druid of power could do it relatively easily as well, without limitations you can pile on enough Magic items to sink a battleship.
Heh, a properly prepared rogue could go in with a +50 or better on his hide checks, use immovable rods and pin the dragon into place... add silence spells target on the rods for countering most spells, and presto - a captured dragon.
The knight riding in for grim'n'gritty combat? Get flight, permanent, on mount and rider. Get both enlarged, grab big lance. Get fire resistance/immunity.
Charge. Repeat. Spring Attack w/ a lot of bonus movement nice, but not strictly necessary. Charge, repeat. Wherever dragon goes, follow, charge, and repeat
Place some limits on this, the tactics will be a whole lot more useful. Is there a finite limit of funds available? Is there a time limit? Is there any classes that shouldn't be used?