Firstly, those drives are SCSI (a type of hard drive controller/interface), which isn't very common, so you'd better check that your pc supports scsi before getting one. From the specs you posted, it looks like your motherboard only has SATA, which is another, far cheaper interface. Sata is very good though, and it's what most people use, seeing as SCSI is so expensive.
Secondly, yes i would buy from somewhere else because it is far cheaper than dell, who rip you off with upgrades that you could get for about a quarter of the price. I personally would go out and buy a nice big SATA Western Digital hard drive, but the make is just down to preference

. I can't see that anything will go wrong from installing a hard drive, but if you are really worried, then get Dell to do it. It's up to you, and how confident you feel.
Hope this helps.