Questions about Automatic

I test drove an older FL day cab single axle, automatic. I believe it has a Mercedes engine and don’t know much more.

One of my questions is in regards to the truck(bobtailed) rolling back on an incline at a red light waiting for it to turn green. I took my foot off the brakes to give it some gas and it reved and rolled back not engaging the gear right away. So my question is, is that a normal thing and if not, what can I be looking at as far as repairing it? It’s the same thing when parked in Neutral and putting it in gear.. it takes a bit for it to engage.

I added some pictures of the 2 things that caught my attention. There was oil on both sides of the engine and that one pipe that looks wet was smoking a bit. Other than those things the truck runs smooth and I do plan on taking it to get overlooked if I do proceed on buying.

I have no experience driving an automatic, so I found that weird and here we are so any information is useful to me. Thanks in advance and happy holidays.