my two cents worth: beware!
shooting flint is highly habit forming - much more so than fixed case cartridge or even cap lock. If you start down this road, you may soon be regaling you co- workers with stories involving sharp little bits of stone ... they will think you quite eccentric, but actually it's them - they just don't get it ... poor buggers ...
as a practical matter, you should take a look around some websites and see what is available in your price range. you could do much worse than a GPR - style from Thompson Center or Lyman (it's how many of us came to the addiction).
other stuff you will need ...
eye and hearing protection
patch material
ball
flints
flint leather
cleaning stuff
patch lube (secret recipe available)
targets
staple gun w/ staples
powder
misc. tools (to tighten cock screw and disassemble gun if necessary, for cleaning)
son on and so on ... don't forget kitchen sink
as regards which powder you should buy, I would council restrain until you work up a load which works best in your gun. This will usually mean a 3F for fifty caliber and below, and a 2F for .54 and above, but that's much more a recommendation than a rule.
the best accessory I've ever purchased is Mr. Schoultz method - here's a link:
http://blackpowderrifleaccuracy.com/
read this through a few times before you take your gun to the range ... it is leaned to percussion, but the method also works well in flint.
in my opinion, you don't need 4F to prime your gun unless you are hunting in very humid weather or unless you are a world class shooter... save your hard earned, over taxed, God entrusted dollars for a pound on you usual grade (3F or 2F) and do more practice.
By the way, there are 7000 grains to a pound. But the 'grains' referred to in muzzle loading is a volumetric measurement, so you don't necessarily get an exact equivalent (but it's pretty close).
Free advice, and doubtless well worth the cost