I did a consulting gig there a couple of years ago, so I know a bit about the place. The most convenient places to stay are the Changi Green and Changi Court
condos which are just outside STC (Changi Green is the closest - a 30 second walk away). Rents for a 2 bedroom furnished
condo there run to about S$1200 per month. Other options are lots of HDB apartments between Siemei MRT and STC.
Buses:
To/from Changi airport: 24, 34
To/from Tanah Merah MRT: 2,24,34
Closest supermarket/shopping center: the one next to Simei MRT ~ 15 min walk. There's a Shell petrol station opposite Changi Court that has a 24hr convenience store as well.