So you'd advise just buying a new one and keeping it throughout uni, and then selling at the end? if i want a new one?
I wouldn't sell it unless I needed a new computer, end of degree or not. After four years, I've been peering at 15" MBPs pretty lasciviously, but it's not a worthwhile upgrade, since my current machine is still completely adequate for what I do with it. (And anything that's too CPU intensive for it to chew through, I can usually do on my gaming rig at home.)
If you want a new one and can afford it, well, fine. Hell, get a new one every year if you want. You just aren't going to save any noticeable amount of money if you do so, and will probably come out behind, not ahead.
I already use Dropbox, Skydrive, but if i want to back everything up as well as having a back up of the back up? Ill also need to have a drive to hold all my data as the rMBP is only 256SSD so a external HD? or NAS?, I already have 1tb and 750GB HD available, would apples own time capsule be better than buying a NAS?
A Time Capule is a NAS, and can be used as such, with both Macs and Windows PCs. It's the only NAS that Apple's "Time Machine" backup software supports. (technically. I use a USB external attached to an Airport Base Station and Time Machine has no complaints.)
But if it's your data you're concerned about, better to set up any old NAS drive for cheap and use rsync. I'm not a huge fan of the Time Capsule because I think it's ovepriced, and it has twice as many features as any one person is likely to use. (It should just be a 2.5" HD with an ethernet port and a small ARM CPU that plugs into any router, grabs an IP, and starts running FreeNAS, broadcasting w/ mDNS, and it should cost half as much.)
I'd probably use the 750GB as my day-to-day auxiliary drive (keep it in the backpack, etc), and back up both the 750GB and the internal SSD to the 1TB drive over your network. (attach 1TB drive to one of those USB/Ethernet NAS controller/adapter things, or put the mechanism in a thusly equipped enclosure.)
Then sync your important (school) documents (not your porn library) to the Dropbox. You probably won't ever have more than 2GB of that, but if you do, you can upgrade from the free version. (Amazon S3 could work too.)
So you have a backup of everything, and a backup of the backup of the important things. (You can always download more warez, ripz, and pr0n.)