If you have a 9-speed shifter, going 9-speed certainly makes sense for the chain and cassette. 8/9/10 speed cassettes are all the same width, if the hub you find is 11-speed, you will need a 1.85mm spacer to take up the extra space.

Depending on which XT rear derailleur you have, you might be able to just slap on a 11-36 cassette and have it work, or you might find you need to add a longer b-tension screw, or add a derailleur hanger extender such as a WolfTooth RoadLink.

Older frames with cantilever brakes often have the studs closer together than modern frames, which can limit the width of rim you can run. For example, my wife's 1984 Univega has studs spaced ~63mm apart, with rims that have an internal width 18mm, and going any wider would make it so I could not set up the brakes as well. Some of my mountain bikes from the 00's have studs spaced around 80mm apart, and they have no problem taking a rim with an internal width of 25mm. With your current wheel installed, find some flat pieces of wood/metal/cardboard/whatever to use as shims, hold them up to your rim on each side, and try to adjust your brakes to have the pads come in contact at a good angle. If it works, then the combined width of your rim plus the shims tells you how wide of a rim you could get away with. This measurement would be the external width.