The house we are looking at to buy has a 150 AMp Main breaker box. Can this be replaced with a 200 Amp box to give additional capacity in terms of load? I am planning to build a workshop that will have a 50 AMP sub panel to provide 30 AMP 220 and several 20 AMP 110 circuits

Sure as long as the remaining system is upgraded to all 200 amp rated equipment, including all service entrance feeder conductors and meter.