So my dad has an old Nishiki 10 speed and the wheels are in terrible condition, oxidized rims and spokes, wheels keep going out of true, and it just doesn't seem very safe anymore. The wheelset on there is aluminum Araya's. The hubs are shimano 600 arabesques.

Anyways I want to get him a decent quality wheelset and have been thinking about going to 700C as well because tires are much easier to come by. I've already checked to see if the brake calipers reach and they do so that's not an issue. The front wheel is the easy one to get. But the rear is where I'm at a loss. I'm not familiar at all with how the rear freewheel(?) mounts and what I can use. I was thinking about getting him some Mavic open pros since all he's going to be doing is casual riding and they're a good durable wheelset. But what are my options for rear hubs? Is there any way to convert or work around for this?

Thanks.