Welcome. I haven't personally owned any Delta Armory guns so I don't have first hand experience but they appear to be a a decent brand.
For that price you're spoiled for choice when it comes to AR15s and the polymer bodies can be just as sturdy as their metal counterparts but the lighter weight will be noticeable at the end of the day. As @Rogerborg posted, avoid the cyma 5xx range as I swear the bodies are made from melted down ready meal trays. A shame really as the gearboxes are excellent, I put easily over 200k rounds through mine completely stock with no mosfet with a 11.1v lipo and it didn't skip a beat