For the 1:9 barrel, keep your ammo at 69 grains and below. 77's will be too heavy and won't group well, they may even keyhole past 100. My 1:9 loves the 69grain Sierra Match Kings. As others have said, AR's can get very expensive, very quickly.
More scope may be better for you. What you get all depends on you budget. Vortex makes the Strike Eagle scope which is an good mow cost 1-6 scope. ($325 retail) But they also make the Vortex Razor HDII JM 1-6 scope which retails for $1300ish.
The front hand guards will always have some play in them since they are designed to be easily taken off to clean under them. Only way around that is to free float the barrel. Adding a gasket or anything can effect barrel harmonics and throw your shots off. Just free float the barrel and you'll never have to worry about that. AR's are like lego's, you can do 1000 different things to them. Here is a pic of my 3gun competition AR as an example:
It has Spike's stripped upper and lower. RRA upper lower parts kits. Magpul PRS stock. JP Silent Captured Spring. Alpha Sports match grade 1:9 rifle length gas barrel. Samson Evolution free float rail. Adams Arms XLP piston conversion with low mass bolt carrier. SJC Titan compensator. MagPul everything. Harris bipod. Hiperfire Hypertouch 24C trigger. Vottex Viper PST 2.5-10x32 scope. Trijicon RMR offset. Etc, etc, etc.