The 1951 Mauser supreme is a fine and sought after action.It is not a converted military and it was available as a magnum action.While I am not an expert on the market,the action alone might bring $300+
By the looks of the lines on the stock,and the checkering,the maker knew what he was doing.You might try to find a maker's name.Some riflesmiths stamped it near the caliber designation.
The .308 Norma is an excellent cartridge.The wildcat 30-338 is nearly identical..308 Norma brass is still available,but .338 brass reformed in a .308 Norma die will work.I think there is a small difference in neck length.
Modern rifles benefit from improvement in technology.The sub MOA rifle is more common today.IMO,that old school custom rifle has something about it that you can't buy easily in an off the shelf modern rifle.
I am trying to say its value is higher than what the market might say it is.
I suggest,go to an old school gunshop with a good reputation with your buddy and the rifle.Ask for an honest appraisal.