1) It does not need to be complete, but you should try to get as much as possible. Note that it's not required to buy all of them. Scour the internet and credit the original rippers. If you wish to offer it incomplete, make sure to tag it as such.
2) First, scale it down to the size you actually need it to be for your description, then play with the quality slider in gimp. Depending on the template used, 150x150 or 200x200 should suffice. If that doesn't work out to well for you, try scanning them with a lower resolution. In most cases, you don't need 1200dpi scans, 300 are usually ok for most purposes, and you won't need as much downscaling.