That Mac will support up to 2560x1600 through mini display-port but I wouldn't try asking the rather anemic Intel 3000 graphics to actually perform at that. stick with 1920 by 1080/1200 (I'd go for the 1200. the extra height is nice for reading/working on documents).
http://www.everymac.com/systems/apple/mac_mini/specs/mac-mini-core-i5-2.3-mid-2011-specs.html
I'm a big fan of NEC monitors. they are generally better built, more user friendly than any others and offer awesome warranty support. they also generally cost more, but, you get what you pay for. I know Dell is popular here but their quality control is notoriously uneven so you could get anywhere from a great to a terrible monitor.
if you just want to spend as little as possible, I can't recommend anything specific. there are dozens of options. most all the panels are made by, what, just two companies now? there wont be much difference between any of the under $200 offerings. watch for sales (make sure it's at least an IPS monitor) and then do some online research (Amazon, Newegg, BH user reviews) to see if that is a product you can live with.
probably want to consider just as much who you buy from, regarding the ability to return it if there are issues. saving $20 by buying from some shady online operator or Amazon Marketplace seller wont be a savings at all if you end up with no recourse for a damaged or defective product.
this site is pretty useful in these kinds of decisions,
http://thewirecutter.com/reviews/best-24-inch-monitor/
all kinds of Dell love there right now but I'll stand by my NEC position based of years of experience kitting out video, graphics and design shops.
