Sorry people, my response got verbose...
I like your ideas of the pistol grip stock, the side-saddle, but the sling swivels...
SLING SWIVELS. While I agree with having a sling of some sort, look at the latest slings on the market [here's one source, below].
http://www.spectergear.com/
Many of the tactical slings don't require sling swivels, at all, but may require some type of adapter plate, which may/may not come with the sling.
Buying ammo and using it up and repeating. I've got to say, this saying annoys me, but I actually agree with it. It annoys me because it's a bit vague and really doesn't address why somebody might change his personal opinion as to whether an accessory or modification is no longer wanted or needed. I don't agree with shooting a lot without the add-on accessories; shoot with your desired accessories ON the gun.
Buy the pistol grip stock [I like the Speedfeed brand, and settled on the short one, the IV-S, after buying the longer one with the spare rounds stored in the stock...a potential waste of $65! The long one went to a brother as a gift.].
Buy the Tac-Star side-saddle [I bought the 6-rounder first then settled on the 4-rounder, a potential waste of ~$20. The 6er went to the same brother as a gift.]
I lucked out with a Wilderness 3-point sling and kept it, but did buy several other slings over the years, which, luckily, I was able to use.
Now buy 100 1oz. slugs and 100 '00' buck. Practice, not just blowing away ammo, but loading from the side-saddle; perhaps taking a class. Try different brands of ammo, if you want, but get comfortable with whatever you choose and get used to it so you know its point(s) of impact and patterns, at various distances.
I.e., loading from a side-saddle is different from reaching into a box placed on a table. Do you put the rounds up or down? While I like pistol grip stocks, maybe I appreciate more as I've but many 100's/1000's? rounds down range with a regular stock. Now more specifically, how's manipulating the safety using the pistol grip vs. the straight stock? How's your hand fit using the pistol grip? The sling! How do those things even work? Do your eyeglasses, pens, pencils, cell phones, jewelry, ear protection, hats, etc. get in the way? How about switching from right to left hand? Standing, kneeling, prone, moving, climbing?
Shooting is practicing a skill; training can help so you're not practicing a bad habit. You need to buy some of those accessories AND shoot with them to determine how those parts will work or not work for you. I don't recommend shooting a lot without those accessories, but with those accessories on the weapon. However, if the accessory ends up not working for you, get rid of it.
PS: consider NOT buying the heat-shield. I just don't see anyone needing one.