Congratulations on your purchase. The subject area is rather broad, however, as each accessory generally comprises a thread in themselves.
Fore-end, Light: Many people like the police or L.E. short fore-ends as there is no over-lap with the ejection-port/receiver; also they allow for the sidesaddle mounting with no cutting of the larger fore-ends. Speedfeed, Remington, Hogue, and probably others are on the market.
However, if you're going for the $200+ Surefire weapon light, then you've already decided on what fore-end to get. Decide on your HD threat scenario and plan/build-up your gun accordingly, i.e., might you be fighting at night? Food for thought: Could your Surefire weapon light fail just when you need it?
Stocks: You mentioned Speedfeed for ammo storage. Here are the considerations. Speedfeeds are great stocks, btw. Speedfeed has two which hold ammo. Both only come in the 14" LOP. If that LOP is ok w/you, then decide on whether you want a pistol-grip type or a straight stock.
Mag Extensions: I think you get what you pay for. My extensions are Remington and came with my guns. They came with barrel/extension clamps, which some of us recommend, yet others don't bother with them. I believe the Choate brand comes with a clamp, but the Scattergun Technology and Vang extensions do not, although I believe there are univeral clamps on the market which should work (see MichaelsofOregon.com).
Sights: Again, plan your threat scenario. Many people prefer just a bead sight. Others, like me, like ghost ring sights. My threat scenario goes beyond the confines of my house. Accordingly, I regularly practice with slugs out to 50 yards, with 100 yard range accuracy-capable. I have my GRS's adjusted to my point of aim, rather than holding low with a bead sight at 25-50 yards. Still others go with red-dot optics. This is a big personal preference area.
Barrel work? I'll refer you to:
http://www.vangcomp.com/
for this issue. I understand the back-boring work can significantly tighten-up your 00 buck patterns improving your shooting distance confidence out at 25 yards, whereas, I'd need to be converting to a slug, which takes time. I believe Vang's work starts at $185, depending on options. For more barrel and sight work, you could be into the $600 range, just for that. Some people swear by his work, whereas, I've never heard anyone swear at his work. I prefer 18.5" barrels vs. 20"'s. I might even like 14" barrels, but that's not an option for me.
What else? Ammo, slings, storage, training, books, safeties, transitioning to handgun, flashlights, moving in close quarters, how about a rifle instead?
P.S., That Surefire light is nice to have when you need it, but it sure does add some extra weight to carry when you don't.