Central heating chemicals

A question for any plumbers or heating engineers reading this:
I have recently drained my central heating system (so I could move a rad and some pipes) and now need to refill it. I have 4 rads, and a 3 year-old Viessmann combi boiler. My question is what chemicals would you recommend adding to the system? Scale and corrosion inhibitors? Any product you would recommend? I live in central Brussels (hard water!). Thanks.