I agree with Tony. I have one of these purchased from eBay:
External enclosure caddy 3½” SATA, switchable between USB2 or eSATA - eBay item: 110236856898
Not the cheapest but it is finished in piano black gloss with polished steel ends, so it looks great and performs perfectly.
You can switch it between USB2 or eSATA interfaces. eSATA is virtually as fast as an internal drive and if your motherboard doesn't have an eSATA port, you can buy an eSATA riser for a couple of quid from eBay too.
Then just buy a good name SATA2 HDD to fit the case. You should be able to get a 500GB drive for around £50 + delivery.
Regards: tf1