Never really heard of Interarms, sorry. To me the price seems pretty good but with no reputation it will be a gamble. Chances are if you aren't abusing it it will work, but you could spend the same or a bit more and probably get a more reputable brand.
You can get a 20" flattop upper
here for $337, M-16 BCG
here for $140, and a complete lower
here for $240...that adds up to $717.
Or you could go
here and get the upper for $399. That would put you at $639 for a Del-Ton upper and DPMS lower. Even cheaper if you did a kit for the lower. I trust PSA a lot more than Del-ton, PSA is mil-spec and although relatively new to the scene has a solid reputation. Del-ton is known as a budget brand, as well as DPMS, both are hit or miss, but at least they are KNOWN in the first place, and a lot of people swear by them.
If you do go with the Interarms, report back with how it goes. I am the type that won't even buy a CAR in the first two years of a model's new generation, so they can work out the kinks, so I am always hesitant about newish/unknown companies. But it is entirely possible (and even I dare say, likely) that they could build you a good gun and if so you should report that...that's how companies slowly start working their way up from relative obscurity.