no please don't sell your d50, it's such a wonderful recorder!
the zoom might be good (i doubt it but it is possible), but it's not worth it to sell your d50. if you were a freelancer and pay taxes and do investments you've probably already would've paid of the d50.
if i were you i would invest in a clean and simple 2 channel preamp and a good bag. buying gear is an investment of money and time. don't throw away your time and knowledge of the d50, it's a decent piece of gear.
edit: in regards to being a student and audio interface, i don't think the zoom drivers will be a lot better than the onboard drivers of your computer (are you running mac or pc?). a good preamp or portable mixer will be a better investment in the long run. even a student.