Thanks!

To get the highest reach => simplified

To get a good reach with better purchasing power => traditional

Best => both

Is this right? And with which would you start, higher reach or better purchasing power?

I wouldn't characterize the languages in those terms. It really boils down to who your site needs to reach. If it needs to be read by people in Mainland China, simplified is the way to go. If it needs to be read by people in Hong Kong, Taiwan, Singapore, and pretty much anywhere else, it should be in Traditional Chinese.

Preferred language is tied to regions, not purchasing power. What I mean is that it's not something like "the rich learn Traditional and the poor learn Simplified", it's kind of like whether a person grew up learning the English spelling of a word or the American one. Except the differences aren't as subtle as they are in English.

If you're not sure which geographic group you're trying to reach, you should translate the site into both Traditional and Simplified. However, don't expect the same translated site to reach both groups, especially if you're going to be using tech vocabulary since that varies even more from place to place.

Singapore and Malaysia reads Simplified Chinese and not Traditional Chinese.

Hong Kong, Macau and Taiwan reads Traditional Chinese. While simplified Chinese is getting much more popular in Hong Kong, some of Hong Kong's traditional Chinese is meant to be read in Cantonese, a dialect. So it's not so straightforward.

I doubt there is a correlation between Traditional/Simplified Chinese and purchasing power.

This is like saying "I need to construct an application for the Iberian Peninsula. Educate me: should I do it in Spanish or Portuguese?"

Answer:

Portugal -> Portuguese

Spain -> Spanish

Your question is not "what script should I pick?". It is essentially "what country should I pick?"

Also, there's Traditional Cantonese Chinese and just plain old Traditional Chinese. The vocabulary and grammar differ.