Of course a ball/bullet starter. I assume you get a ram rod (Although two of my inlines came without ramrods and have no provisions to hold a ram rod)
will you be shooting loose powder or pellets(expensive) If loose powder, you will need a powder container(flask or speedloaders) and a powder measure, preferrably adjustable.
Whatever primers the gun takes, and cleaning equipment. If the ram rod doesn't have a proper jag for cleaning you will need one (Watch the threads, there are four or five diffferent threads for jags, get one thazt fits the ram rod). Lots of cleaning patches. Some bore cleaner of somekind. whether you use hot soapy water or a commercial cleaner
Tools to clear the gun when you accidentally dry ball. (ram a bullet in and forget the powder) A gun grade screwdriver, pliers, etc. Many guys prefer to have a range rod for loading and cleaning. The rods that come with the guns are almost always too short for range work. You may already have a stoudt cleaning rod for other rifles and shot guns. (again watch the threads for the jag. )
Perhaps a small vial of alcohol to clean the oil from the gun before loading.
There is the list of essentials (starter, powder containers, cleaning jag) and the list of really handy to haves (extra rod, gun screw driver, tackle box to hold supplies to and from the range, and lastly the one in three years useful things that may be nice to have. You can easily get by with 30 bucks worth of new accessories, but the gun shop clerk can easily sell you $200 bucks worth of accessories, expensive lubes, expensive bore cleaners etc.
Just as a word of advice. until you are accustomed to loading and shooting a muzzie, keep your powder loads moderate. say about a 70 grain or 80 grain load. That load is more than enough to kill ANYTHING in PA, except maybe in a zoo. Manufacturers give lists of loads. The max is usually less accurate and certainly more recoil punishing than a reasonable hunting load.
And for God's sake, don't exceed the max load. Some folks get the really strange idea that if some powder is good, more is better. More powder can be dangerous. Heqavy recoil usually leads to flinching and that leads to missed shots. Frankly, 1870's Buff hunters out west made one shot kils at 400 yards with 70 to 90 grains of powder and 50 caliber bullets. Also, more pwder doesn't really extend range with a light bullet. Air resistance and short bullet length will cause most in-line bullets to start losing stability past 150 yards. Reducing the powder and using a longer bullet extends range more effectively (Yeah I know, it sounds backwards) For instance with 65 grains of powder and a 500 grain bullet, I can knock down steel rams at 500 meters. Most 50 cal in=line bullets can't maintain stability that far. So for all the fancy pellets and expensive composite felt/plastic bushed bullets, they, for the most part, just don't have the same range, as a longer heavier bullet launched by less powder.