Here are the articles I talked about above on bullet/caliber selection:
http://www. luckygunner .com/labs/brass-vs-steel-cased-ammo/
http://www. ar15 .com/ammo/project/Self_Defense_Ammo_FAQ/index.htm
Please remove the spaces I added to the web addresses to keep them from being a direct link to another sight.
I would be glad to answer other questions and back up my suggestions with documented research in addition to my opinion.
On the Ruger Mini platform now that others have mentioned it there were serious accucacy problems earlier. They have retooled and are reported to have improved. The base price is still around $750 at Wally World (Ususlly the lowest everyday price for a rifle) and for that price you can outfit a very nice AR.