I've had a quick Google which returned the following machine for that model number;
http://www.johnlewis.com/apple-imac-mf883b-a-all-in-one-desktop-computer-dual-core-intel-core-i5-8gb-ram-500gb-21-5-/p1533105If someone hasn't purchased a Mac before then the initial cost does seem high, that being said the Apple Mac products really are solid machines which tend to last the test of time.
The Mac Minis can suffer overheating issues if there is poor ventilation around the machine but the iMac should be fine, the initial investment with these is high but it should last close to a decade so long as there are no catastrophic hardware failures and no need to use high end processing power (gaming/video editing) a few years down the line.
The hard disk drive size is not that large, they don't tend to be unless you go to the pro ranges or upgrade during purchase via Apple or whatnot, so I would suggest a purchase of an external Hard Disk Drive to use with Apples own Time Machine Backup Software (free and included in the operating system software) or an aftermarket solution like ChronoSync (I use the latter to schedule backups to numerous hard disk drives which works well).
I don't think you can go too wrong with a Mac these days, the one you're looking at seems fine unless you're wanting to do things which need a lot of process power.