ALWAYS get a parts book & a shop manual before doing much else.
I would do a full maintenance, by the book, in the order that the steps are published. In that process you are BOUND to find other things needing attention/replacement.
THEN place a complete parts order. You'll also save on time and shipping costs, 99% of the time.
British Only's website will give you a budget price, then you can check ebay. Steadfast & Lowbrow are on there, as well as a few others that do regular business. You don't say where you are, so I have no idea who would be local or close to you. Raber's, Hutchinson, Big D, any JRC reseller, etc...
Oil tank should be approx. 2/3 full in regular running. Previous owner may have overfilled it after it "wet-sumped" while sitting, thinking it was low. Oil migrates down into the crankcase sump, then floods the tank on startup, and can overflow/spew. Always check oil level AFTER running the bike; as long as you have SOME oil (at least visible above the oil intake sump filter) in the tank.