Hi, the Aristocrat you purchased is a student level horn.  There is some debate about green corrosion, and whether must be removed.  I have read a lot of the discussions, and I believe that it should be removed, it does not act to prevent further corrosion, and it does attack the brass.  If your horn came with a neck, I would probably recommend having it acid bathed (as long as there are no other major issues like a repad in the near future).  But the neck issue to me makes this a bad way to go.  Properly fitting necks are not easy to find, and those that have them want a pretty high price for them.  I have not seen any necks offered for less that about $200 with many at a higher price.

I think your $300 could turn into $500+ pretty quickly.  In my opinion, you would be better off to get a good used horn like you mention at a reputable dealer that has put the horn into good playing condition, and I stress the good playing condition part.  I have seen some good student altos around $500 at some dealers.  I would not recommend buying one off Ebay since you will pay the $500 and then likely be looking at additional repair work.