If you don't mind fiddling around inside your new computer (noting that it's often far harder to get inside a Dell than it is with other makes) you could, of course, simply install your old drive into your new computer as a secondary drive. That would save you the cost of buying a USB caddy. However while modern SATA drives don't require you to adjust jumper connections to set them as 'primary' or 'slave' (as you had to do with older IDE drives) there can still be certain complications when installing a second drive into a computer. That's why I suggested using a USB caddy (which simplifies things).
Buying from eBay is no more difficult these days than buying from Amazon. Indeed, it largely operates as a rival to Amazon, with over 90% of items sold on eBay being new 'buy it now' sales, rather than secondhand items sold by auction. Even when sellers specify 'payment by PayPal only' you simply enter your credit card details as you would on Amazon and PayPal handles the transaction (with the seller paying PayPal's fees).
However, if you'd prefer to buy from Amazon, this should meet your needs:
Amazon.co.uk User Recommendation
(NB: Remember that, as per my link above, you'll need to 'take ownership' of the drive before you can see all of the files on it).