I just bought a used HK-91, which is my first HK of any type.  It came with a wide handguard, which I would like to replace with a slim or tropical one.  The barrel will also probably be a candidate for replacement at some point.  I was wondering: Can these types of upgrades be done at home without gunsmithing?  What tools, if any, would be needed?

(PS. The gun came with no manual, but I just ordered one.  While I am waiting for it to arrive, I would like to know what else I may need to maintain and upgrade the rifle as mentioned above.  Thanks.)

You don't need anything special to maintain the rife. The forearm pops of easily via the push-pins. As for the barrel, well, you'll need to have someone VERY familiar with HK guns do that. It's not a just a "take it to the local gunsmith" type of job. Whomever you choose, make sure that they're a trained HK gunsmith.