Of course they are going to say to have it repaired by an authorized agent. That's how they make their money. If it's out of warranty then anyone with a bit of sense can repair them. They are built up in China from generic parts. so are easy to source
I made one of those under the guidance of my best mate who was/is a highly skilled restoration carpenter. We decided to split it in half. I did the top and he did the bottom. I was told to use hand tools and took me hours. He just threw it onto a mortice machine and did it in five minutes!
25l is nowhere near enough to run an air sander, They are very air hungry. 50l with a good cfm will just about do it. The best seller where I work is the Airmaster Tiger 16/550. great bit of kit.