Usually the hardest part of changing a head gasket is getting the heat shield off the exhaust manifold, but, as the garage has already done it that's one stumbling block you won't have to deal with. HG isn't a difficult job, except for the aforementioned exhaust manifold heatshield, as long as you have the right tools. For the head bolts I use a 16mm spark plug socket as it is 6 point and 1/2" drive so you can give it plenty of grunt.
As for doing just one or both. While you are at it, you may as well do both, it'll take longer and you'll have to deal with the heatshield and the bastard to get at rear exhaust manifold bolt on the driver's side (taking the wheelarch liner out and going in from there makes that easier, might make the heatshield easier too, at least you'll be able to see what you are doing). You'll be surprised at how quickly it will come apart. Follow the steps in RAVE and the head should be off in 3-4 hours, less for the second one as the inlet and stuff at the top will already be off. Cleaning up faces while waiting for the head(s) to be skimmed are likely to take longer if you do it properly.