I don't know which model you're referring to but I'm guessing it's a 16" carbine. Probably the CAR A4. You could take a look at Bushmaster's new "O.R.C"(optics ready carbine) as it has a considerably lower MSRP than S&W's M&P15T and then spend that extra money on what you want to add.
Or you could go with the S&W M&15T and get it all in the package.
As for selling the pistols, only you can answer that one. I personally think you should round out your stable. Replace those two plastic bottom feeders with another long arm.
Your P99's fill the same niche after all. But that's entirely your call. If you think you'll regret it then maybe you should wait.
The good thing about the AR15 is that magazines and ammunition are both more abundant and considerably cheaper than those for your FN. The AR15 is also more accurate and powerful. It's a good-handling, light recoiling rifle. It's a good rifle to hone your rifle skills with.