Threadfin,
By now you have either passed or gone forward on the purchase so this may be of little use. I owned a Model 66 for a few years in .30-06 and was impressed with accuracy, craftsmanship, scope mounting and flexibility that a switch barrel design offers. My only complaint, and it was one apparently borne out by others, is that any amount of foreign material in the action may render the rifle inoperable. In my case dust from some fairly strong winds created a malfunction in mine over a matter of hours. Honestly I did not think, as I had experienced the same conditions with other rifles, that conditions were such that I had to worry but I was quite incorrect. The Mauser 66 was designed by Walter Gehmann. Gehmann was a world class shooter for Germany before during and following the war and understood accuracy enhancing design features, however in my experience the precision manufacturing does invite stoppages. The caliber offerings you outline are phenomenal and as a side note Gehmann, apparently at one time worked for DWM and championed the use of the 7x64 Brenneke for use in German K-98 sniper variations, however the already overburdened German logistical chain was not supportive of yet another munition for distribution. The Model 66 is a very good choice and offers that all important characteristic portability- just keep it meticulously clean.

Kindest regards