I didn't realize it, but if your SC is a '77 it already had Weinmann A124 Concaves and if you can snag NOS like-rims for $40@, I'd jump on that.
Not that there is anything wrong with the Sun replacement rims. They look semi-authentic and have even nicer braking surfaces than the originals.

Either way, building your own is the way to go. There are good resources on the web to guide you and you don't need a lot of special tools for your first build - just use the bike to true it up after assembly and you'll be on your way.

Wheel building is easier than it looks, and though you might have to disassemble your first attempt and start over to get things right, you cannot get better by delaying your first foray into wheel building.