how to regulate 18-24V to 12V max 50A?

Basically the title I'm building a 12V PSU 600VA for cost effectiveness I can only get hold of 12V toroidal transformers 6V are ludicrous prices. So after rectification etc I think the voltage will be way up. I want to use switch mode but can I do this safely without it costing more than a 6V 600VA toroidal?

Why not just buy a 12V 50A SMPS?

After rectification your 12V DC will be about 14.8VDC ((12*1.4)-2) with ripple being significant unless your capacitors are enormous. The valleys in the ripple will make most regulators fall out of regulation.

[edit]Your 12VAC will produce about 14.8VDC etc....