multimonitor setups work best with the same type of monitor in the same resolution and same physical size. identical monitors are ideal.
if you change panel types then one might look alot better than the other.
if you change size things look really strange when you drag across or are referencing one screen to the other.
if you change resolution things really get strange!
the only case where i would say two differently sized screens aren't too bad is when one will be your primary monitor and the other is only going to be used for skype, surfing the web, internet radio, etcetera... or when you dont plan to span anything between the two screens and are okay with the idea.
with that said...
8bit ips panels (p-ips, h-ips, s-ips) are always what i recommend but in your budget you might only be able to get a 6bit e-ips panel. this essentially is the same color quality as a tn panel but has a much wide viewing angle. they are only available in 60hz though.
some people rave on 120hz vs 60hz but personally i dont see a difference big enough to rave about. in any case to get any decent difference you will have to upgrade your hardware more often to be able to run new games at closer to 120fps. sure you might see a difference at 70, 80, 90fps but it scales as you get closer to 120fps (120hz). also tn panels have a poor viewing angle unless they use a coating which helps out slightly. personally i try to steer clear of them.
as far as larger monitors....remember that as you increase in size while staying at a static resolution (1920x1080) things will look not as crisp. not saying that it is a bad thing.. i play on a 40" tv and i'm happy with it. just stating that you will notice changes.
of the brands you mentioned... i'd trust the dell as i've heard alot recommendations for it.. maybe the asus. never heard of iiyama you would have to do alot of research on it first.