230VAC to 60VDC circuit
Ebay, telecom 'rectifier' unit, maybe the cost of a few beers, job done.

48-54V is a very standard bus voltage in the telecomms and server game, and you will be able to pick up a used suitable supply for not a lot of money. Of course you can design a 50V supply from scratch but at 750W it will cost you more then just buying a used one.

Note that your dc-dc unit is almost certainly intended to run from an isolated supply, so you will need a transformer in there somewhere, either big iron or a switch mode one.