Well a standard long action has an opening of 3.8" and that seems plenty long. As far as mags are concerned I am uninformed. A 338 Lapua Mag action will have the length you need and a mag that will accept the cartridges you say you want. Now as far as cheep goes, I'm guessing that is a relative term. If you have had a custom rifle built then 2K may seem quite cheep, for me they both are a little out of range but that's the world I live in. This could still turn into a interesting thread yet.