You should try to get at least one monitor that has native DisplayPort to avoid the hassle of the adapter. If the other monitors you get are identical in size and dot pitch then you can still use bezel compensation even if you mix brands.
16:10 aspect ratio is a bit better for Eyefinity because 16:9 gives you a wider aspect ratio than you need and it ends up feeling a little too short and wide. 16:10 also works much better in portrait than 16:9 because of the narrowness of the middle monitor when rotated.
1680x1050 has the benefit of being 16:10 ratio and low enough pixel count to not stress your GPU as much, but I don't think there are any with native DisplayPort. 1920x1200 is 16:10 but very demanding with 3 monitors. 1920x1080 is cheaper and easier to find than 1920x1200, a little less demanding, but 16:9.