are the actions suitable for a rebuild on another caliber and what calibers could be built?

Some say it is and have done it, others say the action and metal was never designed for much pressure over what the original 7x57 produced which is in the 40,000 some psi bracket. Jim Carmichaels said he would even have a 98 Mauser action reheat treated if he were using it for a high pressure round like a 308, 243, etc.

rebarrel to a high grade 7x57, if the barrel doesn't shoot

I would make a .308 out of one and never worry a bit. I would have it heat treated, but I am going to do that to any old Mauser any way. But if you don't re-heat treat it, I still would personally not worry.

The gunmaking world seems to be full of more old wives' tales that refuse to die than any other field that I personally know of. The idea that pre-98 Mausers are hand grenades or will set back in a box of shells is one of them. Hundreds of thousands (probably actually millions) of pre-98's are still being shot with modern ammo. Sure there are problems, but when lookled at in context the risk is not that great. You are in FAR more danger driving to the range to shoot the pre-98.

I would not advise "hot rodding" your loads for her, but that is my reccomendation for folks loading for any action. If I want more power, my phillosophy is to go to a bigger cartridge.

Oh, in regards to caliber, I would stick with soemthing that has a max length of the 7x57. Soemthing like 7x57, 6.5x55, .257 ROberts, or the .308 family of cartridges.