The pictures definitely help! I wouldn't be overly concerned about the siding while its a little ugly to have the bubbling but water intrusion isn't possible unless its so poorly installed it blows up in the wind and allows access to the wall behind. There should also have been hose wrap installed when doing the siding to help with insulation. When working on inside renovations I would tear out the lathe and plaster to insulate exterior walls from the inside and update the electrical. It will be some extra money but can be done over time unit by unit. You can do the sheetrock/ drywall yourself just make sure that you put the insulation behind any pipes you come across you don't want to insulate them from the heat but the cold. This should help with the heat bill.
As for water leak concerns they should be your primary concerns water intrusion from the roof and the flashing should be remedied as quickly as possible to prevent extra damage.
Regardless don't get down just take it a step at a time as @Greg Scott said investing is a journey not a trip it'll take time!