Simon,
The original motor was a Monoperm Super and a smaller prop! It was adequate but no more. The original vessel was completed in 1937 and had a speed of just 11.5knots. Remember, back then a lot of cargo ships could make only 8 knots or so.
Many of my models have belt drive as it is quiet, easy to fit and the gearing down reduces power consumption considerably. My battery is a 6v 7AH SLA. It is a relatively heavy boat but quite fine lined so slips through the water easily. The kit is actually wider than than true scale by half an inch or so, presumably to help stability.
I'd think your 5 pole 540 would do fine but would run more efficiently with a 40mm prop if it were geared down like mine.
Colin