Turning Euro wooden pallets into garage shelving with minimum tools
Those racks appear to have been constructed by simply attaching legs to halved pallets. You'll need a way to cut the pallet rails, which appear to be fairly heavy hardwood. 

Once that's done, it's a matter of running 3" (75mm) gold construction screws through 2x4" (35mm by 70mm or 40mm by 90mm, maybe--whatever standard metric size is available in that range) lumber.


Standard construction-grade lumber is fine. See above for lumber and screw sizing. Do not use drywall (black oxide) screws. They're much more brittle and have less bend tolerance.
Probably not. You need to split the pallets. However, a simple high-quality handsaw will do that job.
Yes. Construction lumber is typically spruce, pine, or fir (SPF). 
Yes, two vertical posts per end will work well.
I'd use three or four per joint, spaced as widely as practical (within 15mm of the edge of the lumber is fine) to create maximum diagonal rigidity. I'd probably pilot the holes through the posts and maybe partway into the pallet lumber.