What to look for in Riding Suits? I have been using a riding jacket but it is a bit hot during summer over long trips. I like the jacket because it is very simple, only one pocket for a key and it protects in accidents (or so I think). 

I have now used a so-called "technical t-shirt" used by footballers and armies as underwear. It moves the moisture out of the skin so my body temperature is more stable even with the jacket. I have replaced my cotton t-shirt under the jacket with it and I feel much better with it. I am unsure why but it works, ideas about such layered bicycling welcome.

I have been considering to buy this suit here but I have no experience with them earlier. What should I look for with this kind of suits? And how do layers work here? Which layer moves the moisture, not to get cold etc?
For the wicking action to work you need to wear something that is relatively taut on the skin. Any common jersey should do the job. I and others have had success with light weight wool jerseys like from ibex. 

Generally speaking, cotton is not a good choice for the base layer next to your skin. It will work for a while but as soon as it gets drenched with sweat you'll be miserable. 

Layering is key for colder weather. First knee and arm warmers, then a cap under helmet, and shoe covers, then a lightweight jacket or winter training jersey. After that, a "dicky" for your neck and tights over the bibs