If you already know you like the platform I wouldn't waste money on a "Starter" gun. Get the best one you can afford.
Or do this. Buy an Aero M4E1 stripped lower. Buy a decent mil spec lower parts kit. Get one that comes without the pistol grip and trigger because those are going to be mil spec and you will change them. So far you're into it about $130. Now buy a carbine buffer kit. That's the spring and the tube and the endplate. That's another $40. Now pick you out a stock. I suggest a SOPMOD, but any of the Magpul adjustables are going to be good. That's another $40-$90 depending. Now you can either get a drop in cartridge trigger like a JMT or a mil spec type like a Hiperfire EDT for $100 or you can start with a nice PSA EPT trigger for about $40. Next get a grip ( get a grip man
) BCM Gunfighter or one of the new Magpul plus grips is a good one. That's another $40-ish. Now you're somewhere between $300 and $500 depending on the trigger, grip and stock you chose.
Now follow me here. This is the turning point. Assembling all this stuff. The good news is you already have most of the tools you need and me and plenty of other guys here, along with a few helpful links, possibly the one from Pew Pew Tactical and you can have a complete lower, ready to rock in less than an hour and a half. The bad news is you will need some roll pin punches and an AR wrench. That'll set you back maybe $30. Then more good news. You now have a high quality complete lower receiver you built yourself for less than $500 and you managed to pick up a new addiction.
Next you need an upper. An Aero M4E1 upper is about $400. And BAM, now you have a high quality AR-15 for about $900. Add a Romeo 5 and you're in business. You can shave off a few bucks if you need too by sticking with a lower parts kit that comes with the trigger, stock and pistol grip. And you can change those out later. The last one I built I bought a PSA EPT lower kit that came with the B5 SOPMOD stock, B5 P23 grip and PSA EPT trigger and I think it was around $200. 10MM might remember the exact price as he bought the same one, the same day.
You could also get a decent ( Complete) upper from PSA or BCA in the $200-$250 range if you want to shave some of the cost. But personally I think the Aero for $400 is a hell of a nice upper at a perfectly reasonable cost.