You are right to prioratise, especially in favour of an external display. Of course, you can sell the original HDD on Ebay, so any HDD upgrade will not be much of an expense.
Having said that, you can always replace the HDD, but having an external display will make things just so much nicer!
Do you think that Dell display is reasonably good for £209? I'm looking for monitors that are around £200, with a 22" or 23" screen, and maybe a USB Hub or Web Cam. The Dell fits all of my criteria (except built in speakers) but would it be a good external display to use with a 13" MBP?