I personally wouldn't install anything other than this quiet fan from Manrose. I can't stand when I turn on a bathroom light and then it sounds like there's a hairdryer going off.
Get yourself one of these fans. The QF100TX5CON is IPX5 rated, which means it be installed in Zone 1. Eliminating the need for a low voltage fans to be used in that zone. No faffing and finding space to install your transformer.
£42.00 inc VAT