I am considering putting together my first AR lower. I plan on purchasing a RRA lower receiver that I see from time to time for $99.00. There seems to be large swing in pricing on the RRA receiver. Is there a grade A, B and C or all RRA lower frames the same? I also plan on using a RRA lower part kit with national match trigger for $119. I have not decided on the stock. This is just to see if I can do it and not so mush to save money.