I want to build an off-grid MIG welder that can run from a vehicle electrical system. What's available in the vehicle is a nominal 12volt battery capable of delivering short bursts of several hundred amps, topped up by a 120 amp alternator. What I need to get to is a stabilized, adjustable voltage around 20 volts delivering bursts of around 100-120 amps.

Is this feasible?

What sort of challenges can I face when scaling up a switching regulator to this sort of power?

To take a different approach, can smaller regulators be paralleled up to create a redundant array, or does bad stuff happen when you try this?

 

Paralleling is certainly an option and common for larger supplies.

Have you seen Aldi's switching welder? Might be a place to start rather having reinventing the wheel.