Help with new router purchase, please.

It sounds like you know what you want, so I'll just give you the review site:

http://www.smallnetbuilder.com/

They do extensive tests of wifi routers (among other things) and rank them according to speeds at different bands and distances. Do be aware that they mostly test connectivity speeds. They don't give as much emphasis to features or ease of use, and they don't test long-term reliability nor support (warranty, firmware updates, etc). You'll have to use Amazon and Newegg reviews for those.

$200 is a lot. It is enough to buy you the best AC1900 routers. But more than likely you probably don't really need the best. AC1750 routers are about $50 less. AC1600 routers about half the price. And AC1200 about $50. The main difference is in max speed and range. If you live in a small house or apartment, the AC1200 may be just fine. If you live in a big house and have devices which absolutely need max speed, then the AC1900 may be worth it.

Asus, Netgear, and Linksys tend to be the top names. But some of the off-brands deliver as-good performance for a lower price. Another thing you'll want to keep in mind is the ability to install 3rd party firmware like DD-WRT. A lot of times the router is missing some functionality you'll want, but this functionality can be added with DD-WRT or Tomato or another firmware (routers are basically solid state computers mostly running Linux). It all depends on what you want from the router.

Edit: Since you are replacing a router/modem combo, you can just disable the wifi on the router/modem, plug the new router into the old router, and set its IP address as DMZ on the router/modem so all traffic gets passed through to your new router. The one thing to watch out for is that the speed of the old router's WAN port exceeds the speed of your ISP's data connection. A lot of the really old router/modems were limited to about 10 Mbps on the WAN port. Wasn't an issue back then, but is now with many broadband connections surpassing 10 Mbps. If that's the case, then you'll need to replace the modem as well with a newer one.