The water hoses will run inside the van as well so worth checking/replacing all connections. I wouldn't use the water for drinking unless you add sterilising tablets (and then it doesn't taste too good).
I use the water in the tank for washing but take bottled water from home for drinking. I would recommend using the proper hose from camping shops as it it quite thick walled and is stronger than garden hose and less likely to spring a leak in the future.
I replaced all my hoses for less than £20. I didn't use any sealant, just secure the hoses to the fittings with small jubilee clips.
Don't take chances with the fuel tank - replace it or get a professional repair done
Buy a bottle of 'RadFlush' and follow the instructions on the label. Reverse flush the coolant system if you can and then replace the radiator cap.