If I have any doubts about buying a gun from someone I don't know Ill walk away without it. Its good practice to use something like this whether buying or selling. http://familysafeathome.com/yahoo_site_admin/assets/docs/billofsale.218155746.pdf
If the end goal is to SBR it I would definitely go with a pistol version off the bat. As for brands and models AKs aren't my area of expertise so Ill let someone else comment on that.