Can try checking out the Anne Black YWCA but the toilets are shared..
It has all the basic necessities and was clean. About 15 minutes walk from the Mongkok MRT station.

Try using www.agoda.com to find a list of suitable hotels with reviews and sort accordingly by prices..

I used the One2Free prepaid data approximately RM50 for unlimited data for 7 days. Speed was consistent and fast.

http://one2free.hkcsl.com/jsp/prepaid_sim_...ges/charges.jsp

I'm not sure about the reliability or coverage of the gov free wifi.

The prepaid data package should still be available. There's a telco shop at the arrival hall of the HK airport where you can buy it from..

The shop's name is 1010
At terminal 1 departure hall, Level7