You should have read the warning on AOSP download page, if your download speed is about 50kb/s, it might take a very long time (~ a few days?) to download about 10GB of different files required to build AOSP.
Also, if you don't have a Google Nexus device, most probably the image you build won't work with your device. It takes a lot of tinkering to add the support for the very specific hardware you have in your phone.
Of course, nobody can forbid you downloading sources and building the image, but I would strongly advise against it, because in your case basically it's pointless waste of time.