You'll want to check some good Youtube channels for recommandations, because each of these keyboards has their strengths and weaknesses(more or less).
Sanjay C is a good one, though there are plenty of others. A search for
the best midi keyboards will get you some good pointers.
It's all about what you need in the end.
Keep in mind that Reaper has multiple methods of putting midi gear to good use.
For the ultimate in customization, the CSI project(Control Surface Integrator) is likely your best bet.
ReaLearn is interesting, though watch its videos to know what you're getting in to, because I have little idea how it works.
Perhaps the device you're choosing is supported by
DriveByMoss (forum thread) which gets you very competent setups ready to go.
As for Reaper all by itself, it can do a lot as well.
Reaper itself can bind actions to incoming midi or OSC messages. For that the device needs to be enabled in the MIDI DEVICES section of the preferences to send control messages.
Or Reaper itself supports it with a control surface driver. If the device supports the Mackie Control Universal protocol, you're reasonably well set too.
Check Youtube videos on Reaper setups. There's a lot of good info out there and ask questions here if you want to know something specific.