In the past I had a Acorn Archimes and therefore I am interested in what to do today with a ARM processor. Are the devices like Raspberry Pi 3, Odroid C2, Pinebook or Rock64 more for experiments or will there be the possibilty to build a whole Desktop PC around them? How to connect a SATA device, a keyboard, a mice etc. to them. Are there Shops which sell whole Systems?
I can only speak for myself.
The raspberry Pi 3 is best suited for something like a media server. It's limited ressources make it terrible at desktop computing.
The Odroid-C2 is the same, just with better hardware.
The Pinebook I use for everyday stuff, like web browsing, email, SSH etc.
The Rock64 is a beast compared to the other. The 4 GB RAM makes it an ideal candidate for desktop use.
To connect a sata device, I would get a USB3 -> Sata adapter (they can be bought at pine64.org).
Keyboard and mouse is just a standard USB plug, which these devices have multiple off.
As far as I know, ARM boards can not be bought as a "whole system".
But you can buy, the board, power adapter, case and periphials in the same shops, so you can essentially buy a whole system.
I use a Raspberry Pi B+ (of the first generation), however, not with Manjaro ARM but with Raspbian, since Manjaro ARM does not support the old architecture, afaik.
It's really slow, actually. I only access it via SSH, so no GUI is running. It serves as an NFS server (central file storage, a 250GB external HDD is connected for this purpose), torrent client (put the Manjaro ISOs there, when torrent downloads were still possible, I fear, they aren't any more for newer ISOs...) and CUPS server (for sharing a printer in the local network). I once set up Owncloud on the Raspi but it was rather slow due to my limited upload bandwidth, and in particular slow for a large number of small files due to the Raspi being slow and some Owncloud bug, so I stopped using it.
Not exactly what you are looking for, but if you are looking for a relatively cheap, powerful laptop based on arm, wait for the Pinebook pro! https://www.omgubuntu.co.uk/pinebook-pro-linux-laptop-coming-soon
Well, I'm a Planet Computer partisan to the max. Their Gemini does everything the Pinebook does and more, includes calling etc. (though you have to open it up like a laptop to make or receive a call). My Gemini runs Debian ARM and Sailfish (never could figure out if, or how, it supports Manjaro).
The top-of-the-line Planet is called the Communicator, and it's coming out in a few weeks. I can't wait! Have it pre-ordered on Indiegogo.
I'm not sure any Manjaro ARM desktops exist, but you can definitely emulate the experience with the Communicator or the Gemini. In fact, that's how I use my Gem at home. I have it hooked up to a dock, which connects to a small TV as the monitor, as well as a Bluetooth keyboard (I suggest the Apple keyboard) and mouse (suggest the Razer Orochi).
The Gemini can also be used, and in fact is primarily intended, as a pocket laptop. It has its own keyboard, and Sailfish is an excellent gesture-based OS for that kind of thing.