So if I understood you correctly, you do have a laptop which is too heavy, so you want something smaller / lighter. But you are also considering a Desktop computer to do the desktop work like working on your pictures and scanning them?
If that’s the case it depends on what demands you have on the laptop, if you just need it to check your emails a netbook is good enough. Using Photoshop on one of those? I tried that once, you don’t really wanna do that.
When it comes to file handling, it also depends, do you need all or almost all of your files with you on the go? Then get two big USB harddrives, one for backup reasons.
If you need only some of your files with you, and you know which ones in advance, then maybe you can skip the external harddrive and move that on your laptop / netbook.
When it comes to file storage at home. Well, first of, if you’re going to move bigger amounts of data around get yourself a cabled network, a small switch plus cables, best would be a Gigabit switch. You’ll be WAY faster copying this way than using wireless.
So finally, storage. Depending on how much data you have, you should have it on two mirrored hardrives instead of backing them up on an external hdd (which most people forget anyway), so either you have that in your desktop computer or you get yourself a NAS (Network attached storage) with two drive bays so you can mirror your files in case one hardrive fails.
So for your info, I’m not making my money with photography, but i am a bit of a geek, so i do have the Desktop Computer – NAS – Laptop setup.
All my important stuff like pictures is stored on the NAS and when I need stuff on the go i copy it in advance on my laptop or an external harddrive.
I hope that helped you a bit.