I don't see a subwoofer in your shopping list there, so if you don't already have one, yeah, definitely, get the towers. Even if you do already have a sub, get the towers. If you don't already have a subwoofer, then I would say that might be a better choice than overhead speakers.
I think the Polks, are a solid choice at your price point. Even if they are not the last word in dynamics or refinement, they will at least get the basics right. In a surround set-up what you're try to achieve is a continuity or smoothness of the sonic image, from left to right and front to back. So, it's more important to stick with a single manufacturer's speaker line than to trying to mix and match, which is not easy (I've tried) and pretty much impossible for $800.
Besides some of the other brands mentioned here, Elac, Paradigm, and PSB might also be good alternatives. And maybe it goes without saying, but I'll say it anyway, you should always make an effort to audition speakers first if at all possible.
All your choices are going to involve compromises, beginning with cost, so that's just part of the process. Do your homework. Plan ahead. And you could wait, save a little more and buy something a bit better, but that is always going to be true. If your Polk speakers check a lot of boxes on your wish list, that is by definition a good choice.
Hope that helps.