Congrats on your great deal on a great rifle. You won't be sorry.

As for the modifications have you considered this - you bought a rifle without the features you really wanted. It may be possible to alter the Sport to have the features you want but they will either require a gunsmith or make you buy the tools you need and learn to do the work yourself in which case you may end up voiding that great S&W warranty that makes the Sport such a great bargain. I don't know that they would void the warranty or not cover anything you changed but it's possible. I saw a Jeep dealership void the warranty on a Wrangler because the owner drove it off road for a few minutes. It had no damage at all from the off road driving - they just canceled his entire warranty because he did something that wouldn't be as bad as driving up my driveway to my house in the woods. I would want to know about this issue before I started making serious changes like changing the gas tube. That can be a source of real problems and it could be S&W would blame everything on that one issue. I doubt that would happen given the S&W reputation but I'd want to know for sure before I made any permanent changes.

Alternately you could buy a rifle already set up with a free floating hand guard and the pop up sight. The fact you need to change the gas tube to get the setup you want will likely cause you to need that new gas tube. I'm no expert but I do know they make rifles like you want. Yes they cost more but you're either going to pay a gunsmith to change your Sport or you're going to have to buy the tools to do it yourself and you'll end up with a box of parts you won't be using most likely and possibly problems you didn't anticipate. As great as the Sport is there are other great rifles too IMO and you should at least consider whether you want to buy another rifle and sell your Sport and whether there might be warranty issues if you do change your rifle.

Good luck whichever way you choose to go. I do understand people like to do their own upgrades and if doing that is something you enjoy then go for it. But still I'd want to check the bottom line before making changes.

In the immortal words of that great song by the Eagles:

"You can check out any time you like
But you can never leave."