If you can swing the cost, PDCs provide zero upkeep and infinite size hangar space. I'm in the process of building a 500,000 ton "borehole" class storage base, ideally for storing any ships that have a high failure rate but a high value, like strikecraft with no maintenance space but high speed, and dreadnaughts that would otherwise incur a hefty annual cost, not to mention specially captured swarm and precursor ships that can be stored safely. In fact most of the reason for this storage base is to store captured swarm ships.
Hangars prevent all system failures and zero out upkeep - basically cold storage. Ships that are inside a hangar can have their armor repaired via the hangar ship's maintenance supplies, as well as their standard internal systems, and box launchers reloaded.
Maintenance installations draw mineral cost, and prevent failure, a simple form of paying upkeep. Ships in hangars incur no mineral cost.
Maintenance modules work exactly like installations, with the notable point that they must be in orbit of a colony. A free-standing station with maintenance modules will not serve any use, but once put around a system body with a colony, then it will apply towards the tonnage support. This is most useful for mobile fleets, if you've got a swarm of 100 HS destroyers as your main fleet, then you can make a forward base by sticking 25 maintenance modules onto a mothership and then ferrying supplies over via cargo ship. It makes long, temporary guard posts more realistic than trying to shuffle 25 installations over.
If you want a big ship, add an orbital habitat. If you want a mothership sized carrier, add an orbital habitat and any number of other components, and it's built from industry or shipyards (if you've got a big enough yard). I do this for my "megaformer" ship, 1 OH and 50 terraforming modules. Similar for mining modules and sorium. I've debated similar for hangar ships, but I usually don't have a problem getting a single shipyard up to very high tonnage, with just one spillway. Do note though, that you can build military ships with an OH, and make proper deathstars that are constructed at industry and theoretically infinite size - if you can afford the annual upkeep. A PDC built to store a deathstar allows for free upkeep.