Make sure the rims come with bearings, etc. If your going thru a retailer, just talk to them. Make sure your axle size is right. You shouldn't need to change your front fender with a 21 inch rim. But you might want to, just to show off the rim. Also may want to consider dropping your front end an inch or two to get the bike back to ride height.
Some people have had issues with the 21 tire rubbing the fender and scorching the paint. It's a good idea to drill out the 2 rear fender mounting holes to 3/8 and lift up on the rear of the fender when tightening the bolts. Then tape a 5/16 nut to the 21 tire and spin it to check clearance. Will you be happy with a 21 front and a stock rear? That could add to the cost if you are like me. Stock rear wheel sticks out like a turd in a punch bowl.
"If it rolls, floats, flies or shoots, runs on gasoline or gunpowder, goes fast or shoots a big bullet and makes a lot of noise - it's cool."
The Avon 120/70/21 is a lower profile than what Metzler offers and will not rub your fender if you drill out the rear two holes to 3/8" and rock the fender forward (pick up on the back/bottom side of it) as you tighten it down.
If you have ABS, you need the correct front left wheel bearing.