The cleaning solvent will depend on the powder being used.
I want something to prevent it from rusting again
If you do a good job cleaning and drying it the first time any good synthetic oil works just fine at preventing rust during storage.
Ballistol, Eezox, Barricade, Slip2000...you name it. They all work well if its clean and dry before applying your oil. If you want to make sure its dry just give the bore a good shot of WD40, HEET or Brake Clean. They will displace water quickly.
Ultra Bore Coat which is now called Dyna-Tek is another layer of protection. It bonds to the metal kinda like Cerakote and helps seal it. Some people swear by it especially if using T7 powder.
http://store.dyna-tek.com/Muzzleloader-Kit-p/gn-muzzleloader.htm