First, always be careful with those converted rifles. There is NO WAY you can "rechamber" a 7x57 to a 7.62x51. You have to either replace the barrel, set it back so far as to make the rifle possibly unsafe, or rebore the barrel then drill out the back end to install a chamber insert and ream it for the 7.62 cartridge. Only the first option is a good choice. The others can introduce problems that would not occur if the rifle were simply left in 7x57, the cartridge it was made for.
I own 3 Spanish Mausers (one is German-made) and have fired them a fair amount with no problems. All are in the original caliber.
Yes, Spanish-made rifles tend to be soft, but they will still stand up OK within their design specs and really will take a lot more than most folks think. And most of the early Spanish Mausers were made in Germany, not Spain.
But the original question was about using one as a hunting rifle. I answered the OP's question on that basis, not on whether there are better rifles made today, or how much of an overload it would take to blow it up. The 7x57 is a perfectly adequate deer cartridge and a Spanish Mauser in that caliber and in decent shape can be a good and inexpensive hunting rifle.