I've got a cheap dpms rifle that I'd like to add iron sights to. I was thinking about adding a quad rail as well and was wondering if there would be an issue with accuracy if I mounted regular flip up sights on a non-free floated quad rail. I've already got a MBUS front sight, but I can't mount that to the gas block. Just wondering what your thoughts on this would be, and maybe what a good lower end quad rail would be for it.

The Daniel Defense Omega rail will drop in without modification.  It's free floated, so mounting a sight to it shouldn't be a problem.  The price isn't terrible for what you get.

There are sights made specifically for railed gas blocks, but installing a carbine length rail limits you to just 7" of rail for accessories and a very short sight radius.

Swapping out your gas block to a Low Profile GB, however, would allow you to mount a longer free floated rail that will maximize your sight radius.

With prices as low as they are, you might just want to buy a complete URG (upper receiver group) and sell off your old one.  You can even upgrade to a Midlength gas system and Free Float Rail complete with BCG $370
e.g., PSA Mid w/ 13" Keymod FF Rail + BCG, CH