Hey, you are already off to a great start by researching on your own, we like that
First things first, since you said you have been reading stickies, I hope you read all the FAQs and know about our AV system. From what you say, you are over 18, at least I hope :P
I would suggest that getting AVd be your top priority right now, this way you can get a look at our buy/sell and retailers section, where there is plenty of new and used rifles and pistols.
What you are seeking, our retailers have plenty of. I would personally recommend the KJW/TK M4 GBBR. Probably the best for the realism you are looking for, and its a GREAT performer.
Running a GBBR can very very costly to get up and running, Getting enough mags to go a good amount of time in a game will cost the same or more then the price of the rifle. Upkeep of the rifle can be a little more time consuming, but its well worth it.
Welcome to the forums, have fun!