Hello, I think it's 2". I have a transition piece and then a garden hose on it. I use a thicker 2" hose to empty the sludge collector, works great.

Unfortunately I don't know. I used the pool hose, which is about 5 cm in diameter. But there is an adapter at Hornbach that you can screw on and so connect different hoses. The pressure of the pump is so high that you have to fix the hose with a screw clamp. I have not yet found an adapter for a normal garden hose connector.
The pump is super and strong. An additional on/off button would be even better.

Yes, the pump has 2" (DN50). A connection made of hard plastic is also included with 2" or reduction. Greetings