It might be worth thinking about a Mac Mini. I’ve recently bought the middle model and have it hooked up to an (excellent) Dell U2515H monitor. Neither the MacMini or 21.5 iMac can have user upgradable memory (unlike the 27s). Agree on 8GB minimum.
Performance of the Mini isn’t much worse (the Dell monitor is excellent), for a few hundred quid less. You do have to run a script to get OSX not to treat the monitor as a TV – straightforward though.
It’s my first experience with OSX. Can’t say I’m preferring it to Windows yet. If he uses Office for work then Office 2011 for Mac is bloody horrible; at least Outlook 2013 is available now, as is OneDrive Business (beta, but stable).