All you need is a front if you're not concerned about them matching.
Look for a wheel listed as 559mm diameter that's rim brake compatible.

IF you are going to add a rear, you want 135mm spacing.
You'll have to determine if you have a cassette or free wheel hub.
IF you have a free wheel hub, it'd be a good time to upgrade to a free hub, but you'd have to buy a new 7 speed cassette and 4.5mm spacer to use on the 8/9 speed hub.