My budget after I buy the scope is $500
That is a workable budget; but you are going to have to prioritize where you want to spend it and what items are most important to you. I'd also consider buying used where you can to save money.
1. Quad-rail Picattanny handguard
Lots of good choices here: Troy, Samson, Larue, Daniel Defense, Knights, YHM, PRI, etc. There are so many rails out there now I can only speak for a few; but I have always been impressed with the Daniel Defense, Larue and Knights products. The Daniel Defense also offers an aluminium barrel nut that makes the weight lighter if weight is an issue (actually lighter than the plastic M4 handguards). Buying new, you'll spend from $276 (DD or Knight) to $330 (MRFS). The Larue is a nice midrange choice (~$300).
Also consider ease of installation and whether you have the tools necessary to install the handguard. Some of these go on very easy, some will need an action block, vise, wrench, removal of muzzle device and a mount to align the rail with the upper receiver. It isn't very hard; but you will need the tools.
Personal preference mostly. Tangodown is a very popular grip; but make sure the rail system you buy has the depth of rail slot specified by Picatinny (the SIR does not and neither do some other systems). The TD runs about $60. I am using the Grippod; but that is probably outside the budget you have specified.
3. Weapon Light and mount with pressure switch (preferably a Surefire)
This will eat up pretty much the remainder of your budget unless you are able to cut corners on the rail system. About the only Surefire light you could go with that wouldn't totally destroy your budget would be a heavily modified Surefire G2. I would talk with a dealer called BrightFlashlights. They have a lot of practical; but affordable solutions for weapon lights and this is an item where you can end up spending anywhere from $40 to $400.
What do you need the sling to do for you? There are a dozen different tactical slings depending on what special needs you have. Do you train to tuck the rifle after transitions? IF so, that eliminates several "tactical" slings right there. If you don't know the answers to these questions, the safest answer is the Blue Force Gear Contractor's Pack Sling that is basically a lego-set of slings that will let you make a single-point, two-point, three-point sling for 20 different rifles; but it costs about $60.
I assume you just need a flip up rear sight? If so I have owned both the Troy and ARMS. I like the Troy a little better; but the ARMS is better suited to your budget and is a perfectly fine sight that will do everything that needs to be done. Regardless of which sight you go with, use blue loctite on the locknut when you install it. The ARMS #40L runs about $85.
6. Anything else that you think is cool if there is money left.
You'll be pretty fortunate if you have money left. I would highly recommend buying used parts from guys upgrading to the latest new thing as one way to keep your rifle under budget.