Check vavle on domestic side of hot water heater.

[Great-Kazoo]

My worry is my lack of DIY skills and not the cost (although I don't want to pay the $500+ I'm sure a plumbing company would charge to install an expansion tank on Sunday). I've never soldered a pipe and I'm not sure this is the time to start. . I'm confident I could cut the check valve out and use compression fittings to get it back to the way it was.

If you're that confident for the task, you're more than capable of installing the tank

There's no time like the present to learn.

Every home repair and improvement we did started with saying what the hell, have to learn some time.