Disc brakes installation I have got IronHorse Maverick 3.2 bikes with v brakes and I want to upgrade to disc brakes. The fork and frame both has Is mount so they should be able to get calipers on them with the right mount adaptors.

Should it be possible to put 28' (CHECK THAT) disc rotors on them? (The wheels are 26 inch / 559mm ).

If the wheels don't have fittings for disc rotors, do I have to buy complete new wheels, could I replace the hubs only?
Rotor size is unrelated to wheel size, but your fork will have a maximum rated size. Any larger than the rated size will put you at risk of a (literally) fatal breakage. Disk brakes dump a lot of torque into the fork. Have a look near the calliper mounts and there might be advice on maximum rotor size.
A bike of that vintage will probably want 180mm or 160mm rotors. A nominal configuration would be 180 front &amp; 160 rear.
Take a look at your hubs. Do they have 6 bolt holes on the non drive side to attach rotors? If not you need new wheels. Unless you already know how to build wheels, don’t bother with replacing hubs. Seek a pair of disk ready wheels.