[Dragonfire]

Any of the metal second stages with down stream valves are almost indestructible as long as you treat them with a little care. Most of the parts are interchangeable and the LP seats can be home made. The diaphragms are the part that may give you problems in the distant future.
The Sherwood regs have a couple of flaws that can be somewhat of a pain. The air bleed in the first stages can clog up requiring the piston to be replaced. It will not prevent the reg from working; it just stops the airbleed dry system from working which means you will have to clean the reg more often. The plastic seconds had a cracking problem around the inlet hose connection.
For first stages I think would add the Scubapro MK-2 to the list. Simple, very few parts and easy to repair.
The only parts "difficult" to find in either the MK-2 and the Sherwoods are the HP seats and these can be turned over and reused (once ) the rest of the parts are common Orings. Often times the LP seats can be turned over as well.
So if it were me, a MK-2 or SW first stage and a couple of metal seconds with a spare parts kit of Orings, HP seats (the most important part), some second LP seats and a diaphragm or 2. Both of these first stages can be serviced with a large adjustable wrench and a Oring pick....oh and a 2 inch finishing nail.