So I have Civ 5 on the main PC, but I'm saving for a laptop. What I want to know is can I
a. Install Civ 5 on both machines without downloading it (I have a physical copy)
b. Have DLCs on both machines.

a) Yes. Just use the same steam account on each machine when installing. You should be able to use the disc both times.
b) Yes of course. DLC are tied to your steam account, not the original machine you purchase them with.

Awesome, thanks PieceOfMind.