Dude, don't sweat it...like I said earlier, we've all done it, especially starting out.
Bud's LE/MIL has been having the Smith and Wesson AR-15 (MP-15 OR) stripped down (ie no iron sights or fancy furniture) for about $850 lately, and you can get a $100 mil rebate from S&W on top of that price. Before the scare, and even once since the scare, they've had the price regularly at $750, which I would assume will be back again in the next few months if not sooner. Without looking at the Bushmaster vs S&W spec by spec, my guess is that they're comparable with the S&W probably being a bit better...if nothing else at least due to S&W being a solid company. I'm sure others with more AR knowledge can weigh in better than I can.
Go to slickguns.com and see what prices are going for (and these aren't even including the sweet LE/Mil deals) and you can base the market off various different prices. If you're just becoming a gun guy, you'll more than likely buy another AR again as firearms can be quite addictive, especially when you find them at an awesome price. Poor Buddy Spike is having to apply for food stamps because he spends so much on guns and accessories (I kid I kid). M2 is the same way, except for the that he chooses the mighty AK!
As for the buying a firearm online and going through an FFL, the process couldn't be any simpler (unless they ship it to your front door). You buy the gun online, tell them which FFL near you that you would like it sent to (make sure they have the FFL's info, if not, most companies will contact the FFL once you tell them which one), they send the firearm to the FFL (most places have free or cheap/$5-$20 shipping), then email/call the FFL to give them a heads up that they have a firearm for you coming their way, once it arrives the FFL calls you, you go over to the FFL and have the necessary credentials you would use to buy a gun at any gun store (ID, orders, etc), you look the gun over to make sure it's what your ordered with no damages (if by the weird chance there are problems, do not accept the firearm and call the company), then you fill out the standard NICS background form and he calls it in, a few minutes later you pay the guy and walk out with your gun (assuming you don't live in an anti-gun State). Bam, done, got the gun.
FFL's are pretty easy to find, just do a google search or even look on budsgunshop.com and they'll have a list in your area, or even call the local gun store/pawn shop and see how much they charge. Most charge anywhere from $25-50 per transfer, and many give the military their best rates...I currently pay $25, but at my last base I paid $20. As for the tax you speak of...well, unless the Senate Bill gets through to become law, out of State online sales don't directly charge sales tax just like all other online/out of State purchases.
There you have it--nothing to be afraid of and often allows you to get the best price! Good luck!
Edit: Looks like Social ID beat me to a lot of this.
As for your question about still getting the LE/Mil discount, if you are retired they still give it out. Here's the list that Buds uses:
https://www.budspolicesupply.com/catalog/le_info.php