This is an easy one... I keep PVC and unions on-hand so all I need to do is replace the chunk that may be leaking if it needs replacement.
If the joints are done correctly you will not get leaks at all. PVC is under much more stress when pressurized within the walls of our houses than it is under when in the tank. Glue joints should never leak. Threaded joints can leak, but that usually just takes some thread goop or tape to seal it back up. Vinyl and barbs may just need the clamp tightened...
To make sure you have a good first aid kit for plumbing keep a box on hand with the following:
Assorted PVC parts (Elbows, couplers, unions, etc)
Chunk of PVC in the sizes used for your tank
Spare vinyl if you use it
Hose clamps
PVC Primer
PVC Glue
Thread Seal Paste
Thread Seal Tape
Bulkheads (sizes and types used on your tank)
I built my kit by buying extra pieces when building and refusing to return them. That way I knew they were identical to what I bought in the first place. This is especially important for the unions as all brands have different threads.