So what I'm hearing you say Vasilis - is that you have a B2B solution offering in mind that is going to require hardware and software development, and that it is likely to be involved in industrial settings.
Based on this I don't think you need to be "IoT" (which I recently heard aptly described as Internet of Tinkering). what you really are looking for is a developer who is experienced in building embedded solutions in industrial settings. That can include IoT - but unfortunately a lot of current "IoT" experts come from a world of Agile/Internet 2.0/Release To Web methodology that is not very realistic in an embedded systems world.
I would recommend you look at companies with experience in embedded SCADA systems (Supervisory Control And Data Acquisition) which is realistically what "Industrial IoT" or "Industry 4.0" is. SCADA systems have basically been doing for the last 20+ years what IoT claims it will do someday.
What IoT has done for this industry is push the cost floor downwards with everything from cheap RFiD sensors to less expensive "plant floor" sensor systems. They have been working on "Cheap, Light, and Functional" now for some 40 years (they 20+ years is since wireless came into it).
As such that segment of the industry is much more sensitive to security, reliability and service capability than most of the IoT industry
Hi Vasilis, I own an IoT development house. let me know if you would like to connect, and can discuss in more detail