You require 3D scans for deciding for implants. An OPG or x ray cannot decide implant treatment. Visit dentists for clinical examinations followed by 3D scans if required.

Every surgery may have some risk associated. There is a procedure called as sinus lift which may be done in your case so as to accommodate the implant. But do not worry. If your dentist is confident about doing it well and assures that all will be okay then yes go ahead. Dont worry.

Sinus lift surgery can be done first and then later on implant can be done

There is always an element of risk in any surgery. In this particular case yes there is a risk of sinus perforation during the sinus lift surgery which (judging from the opg) is needed here. However the risk is minimal, you should go for the surgery if the doctor has advised for it and be positive about it. The prognosis looks good. To be on the safe side you can go for a cbct(3 dimensional X-ray)report so the doctor can plan it better and is in a better position to asses the risks involved. The final call and risk assessment can be done only by the doctor who has clinically examined you. The X-rays are just an adjunct in diagnosis and treatment planing.