Wow. I had no idea iron sights were so expensive.
Yup. A good set of iron sights need not be expensive to get something good. If you do decide to get the OR, decide on what optic (red dot, ACOG, magnified scope) that you want to mount first. Then choose your iron sights to be compatible.
Do you want folding sights & co-witness them to your dot? Do you want to mount a magnified optic, then a set of 45-degree canted sights?
This way you roll the AR to use your irons, and they don't get the way of your optic.
You can mount whatever you want on the rear. The front sight that you mount onto the gas block will take some thought. Since the gas block can get hot, polymer front sights are not recommended. If you do a mag dump, the gas block will get hot enough to melt polymer. If your pace of fire isn't enough to get the block too hot, you might be able to get away with a polymer front sight.
As to the 15-Sport v.s. 15-OR, it's up to you. They're both great rifles.
OR = forward assist, dust cover, railed gas block, chrome lined chamber & bore, 1:9 twist rate, M4 profile barrel.
Sport = "F" marked A2 front sight, Magpul folding rear sight, straight barrel, Melonite treated chamber & bore, 1:85R progressive gain rifling.
IMO, is a forward assist & dust cover important to you? I'm don't intend the question to be condescending. If the ideal image of an AR in your head includes a forward assist & dust cover and you want them, spend the extra $100 + $ for sights to get the OR. You'll be happier.
Whichever one you choose will become a money pit. Enjoy!