DIY Photo table for Flat Lay

1) What lens to get with Nikon D810? I want to get all those details in fabric of a t-shirt. At the moment I use D7000 with 18-200 lens that came with it. I got white balnce set to custom and colorchecker passport with correct exposure. I take photos in ISO100/F18/1sec. Image is kinda sharp all across but I lack Details, image looks flat (usualy red and black products). For light I use D50 95% CRI lights. And my photos usualy come out flat. Myb because my lights are set 90degree on my "subject" or is my lense crap? Or is sensor in my camera too weak and can't pick up those details?

Image quality is usually a factor of (in order of importance) 1) lighting 2) lens 3) body. Because your subject is not moving, you need to optimize your lighting first. Yes, A D810 is an incredible camera, but even it can't make up for poor lighting.

So is 2nd hand D810 + some lens a good option? They go for around 1,8k€ atm.

I'd also use this camera for "scaning" biger prints.

You might consider a Sony A7R. That's what I use with my "Digitiler" http://maxotics.com/service/  But again, I'd first figure out the lighting and that will probably mean you need some strobes.

2) What kind of backlight is used here https://www.youtube.com/watch?v=ACi0rhurb4E at 1:47 on.

My guess is high quality T10 fluorescent tubes.

Nothing compares to flash/strobes.  I'd look for some cheap ones used and just bounce them off your ceiling or against a white card under your table.  It's hard to get good color saturation with continuous light, at least for photography.

Strobes are so powerful, it's easy to bounce them off anything and still get a lot of light reflected.  Also, they have better color response than any continuous light you can buy.