I know I would like a magpul prs stock on it. I'm going to try and assemble my own upper. I'm thinking about the following. 2 Stage trigger lpk. Milspec flat top upper. Milspec bcg Miad or ergo type target grip. Low pro gas block (already have an m4 no need for irons). I'm wondering if I should go with a 20 or 24 inch barrel? Does anyone have any input on the build?