I think Shanghai is preferable to Singapore for "getting out of the city" as you can take a bullet train quite cheaply to lots of places. Hangzhou is rather crowded, but has nice mountains and obviously the lake. Nanjing is really laid back and relaxed. Both 90 minutes or less by train. Singapore you're stuck to flying somewhere, boat to Batam, or a car to Malaysia. You'll feel just as crowded as Shanghai. The air is better most of the time though.
Hong Kong is the best for the lifestyle you seem to desire, but IMO will be the worst option for engineering jobs. Have you considered looking in Shenzhen? You should have job options there and you're right over the border with Hong Kong.
I can't speak to your possible pay as that is highly subjective. "Expat" deals are possible for anyone any which way you decide to apply, if you're worth it to them.