I have built some rifles based on milsurp actions, or rifles, and I would say that it depends on how much work you are able to do yourself. If you can cut, and crown the barrel, or install a new one, and chamber it, modify the bolt, clean up the top of the receiver, drill, and tap for scope mounts, modify the safety, or install an aftermarket trigger with a safety, etc. then Mausers, etc. make a fun project. If you have to pay to have those things done, I would look for a used rifle in the caliber you like, or in the family of the caliber you like, 30-06 family, 308 family, 8x57 family, etc. Some actions will feed many different calibers (my .308 Ruger feeds .257 Roberts dummy rounds like grease), but most work best within the family they were designed for. As far as caliber, choose the one you like the most, or that you find an action you like for. Both will do the job. ( I did build a 6.5x55 on a 98 Mauser with a manlicher stock once) I shoot a lot of cast bullets these days so that influences my caliber choices some.