aw is a good place to start, I also buy guidebooks for the area I’m going to be in. Guidebooks vary a lot in how they rate difficulty. So you have to get a handle on how each author rates streams. I find my guidebooks by simply searching on Amazon or ebay. Old is okay for river descriptions but access and roads may have changed. If the river is managed by a local agency they often have materials available. Check out their website or call them. Sometimes these materials are very good and sometimes lacking.
Plan on spending time scouting put ins and take outs, and sometimes encountering conditions that make paddling on a given day unsuitable (high or low water, limited access, bad roads, extreme weather). These days are not “wasted” but valuable for their beta.
You need to understand that aw “suggested water levels” vary as well. The information is provided by individuals (stream team contributors) so be sure to read the comments by users at the very end of the river description page (scroll to the end). Often that is where the most recent and valuable intell is (end of page).
Realize that there is no single resource for what you want. Regional message boards like adirondack or northeast paddlers, or southwest paddlers have valuable intel for their regions. Local paddling clubs often have individuals that will share info. Stopping by at governing agencies like park service visitor centers, blm permitting sites, and commercial livery and rafting sites can all be good sources for intel. Even outdoor rec stores can be good sources of info.
Sometimes it is advantageous to hook up with others. Events like Week of Rivers, Rendevous, and some Festivals are also great ways to learn about streams for pfds (personal first descents).
I always try to get a Delorme or Gazzeteer Guide (state road atlas with secondary roads) for the state I’m paddling in. It can really help with the shuttle.
And lastly realize that ww paddling, even class II, is best done with others. It helps to have others to shuttle with and they can provide additional support on the river. You want to develop your skillset so you don’t need to be led down a river. As your skill increases your need to follow decreases and can deal with adversity better. All this is a really nice way of saying if you are willing to practice, increase seat time on familiar streams and take instruction then you may find your margin for error is less important and you become more tolerant of high and low water, wood, and are able to boat scout more of the river.
I tend to look at flow pretty carefully. With two artificial hips the idea of bouncing over lots of rocks on long swim is not real appealing. So consider your physicality.
Lastly, manage your expectations. Oftens things don’t go exactly how you imagined but enjoy each river for its uniqueness.Some streams are “one and done” but most are worth revisiting.