Here are some useful resources which may be useful in your renovation search.
This directory listing provides a useful resource of RRCs. You may check out their registered company details, status as well as any history of infringement of HDB regulations over the past 24 months.
Check out under the Contractors (Renovation and Interior Design) section to check if an interior designer/contractor is currently blacklisted.
A useful resource of information about HDB: history, floor plans, statistics, list of BTO, SERS, EC, etc.