Well just to be sure give it at least half a bottle of Startron starbrite diesel enzyme treatment in a full tank and use the rest on another.
Any nasties lurking will start to get sorted out .
The line out of tank obviously exits the bottom , but where it exits there is a small bell shaped plastic affair about the size of a coffee mug and this sits over the top of the tank strainer… the return line puts fuel back into this as well so any hot fuel goes straight back to engine.
Now if any crusty stuff is in there it sits on the strainer so what some owners do is swap return and fuel in line over up in the engine compartment where the rubber lines join the underbody steel ones .. this can be used as a get you by and works quite well but keep an eye on the pre filter because if anything is floating around in this bell some of it will end up getting caught altering you to what’s in there.
Removing the tank sender unit and having a peek inside the tank and the general state of the sender unit will tell you a lot. Use a mobile phone and torch and make a short video clip to see what’s happening in there.