Being new myself, I don't have much to offer on the topic. But I can tell you a couple things.
Never by a cheap gun, unless the purpose is litterally to just wreck it. By buddy got one from Crappy Tire a while back, and I feel like I could snap it in half right in my hands. Ignoring that, sizes up well, so that's good. Full AEG shooting a whopping 200 FPS! WOOOO! So yeah, never buy cheap.
Next, as far as I've read around the place, people don't recommend you buy any upgrades until you've been out gaming a bit. You know, get a feel for the sport, your gun, etc. If after the match you feel like having a foregrip would have helped you, pick one up! Same with sights. It was recommended to me that I just use the Irons for a while. I figure I'd see how it all faired out, and go from there. Most videos you see, people tend to have a red dot of some sort. But everyone's different.