One that has a native resolution the same as that you intend to use.
Indivision with Amiga HiGFX back is 1024x768.
I dunno about whole number of pixel scaling (e.g. 4x 320x256 = 1280x1024) perhaps that would be a better native resolution. Might be worth try before you buy? Or get one you can return if you're not happy.
Indivision can support 50Hz but can also convert to 60Hz albeit at the sacrifice of smooth scrolling.
I'd recommend at least trying to find one that supports 50Hz vertical refresh. (Note: 56Hz is not close enough!)
I'd still recommend using a CRT though.