Jonathan, I use a 27” NEC and a 23” Asus. I use only my NEC with LR, since I didn’t like the way LR used two monitors. With PS, I open it on my Asus and then drag my photos to my NEC. (Depending on your system, color management may be a little tricky with that set up.) The only slight glitch with different size monitors is moving your mouse pointer between the two. On mine, my mouse won’t move across from the bottom part of my NEC. I’ve got used to it, but you need to be aware.
I got my Asus recently from Newegg. I just picked one of their most highly rated (by customers) monitors. I think it was about $150, and does what you’d expect. (Oops, I remember that I actually bought my monitor from B&H, but chose it based on the Newegg rating.)