- I'm looking for ways to improve my lighting and my overall composition.
Tips on composition are outside of the scope of this site, as regards to lighting technique then that's bordering on too broad a question also, as the flickr account you link to contains many images with different lighting styles and techniques. Maybe you could pick out a single image that you want to emulate?
The only thing I can say in general is that a lot of the images contain a volumetric component to the lighting, which is achieved by lighting through some cloud of particulates (smoke, dust, haze etc.) I recall a comment from the photographer, Avanaut that the Hoth images were accomplished using reacted plaster of paris dust and an old fishtank full of water. The water helped suspend the dust for long enough to get a photograph.
- Is it necessary to have expensive equipment to do such stuff ?
No. You can get by with DIY stuff for this type of work. The only thing I'd spend money on is a good tripod, preferably with a geared head so you can adjust the camera angle very accurately.
You can pick up other supplies (lights, old fishtanks etc.) second hand pretty cheaply and they will do just fine.
What you do need is experience and a lot of practice. I first became aware of Avanaut with the Hoth images, which were created in 2009, you can see a progression in the complexity of the lighting over the last 5 years, and you can imagine how many shots were taken and never posted to flickr over this period.
- Can those results be obtained only by editing the pictures ?
Excluding the shots where models are superimposed over standard outdoor images, most of the work has to be done with the lights. Post production is not strictly necessary but adds that layer of polish, giving nice deep blacks, contrasty edges and rich hues to the coloured lighting queues.