New car (to me)

Hey guys,
Allhamdullilah I just recently just got a new and first car, it’s a 2017 Nissan maxima sv, with 145k driven.

I’m still in college and this is still like a house car but it’s mainly mine, so I really want to take very good care of it,
The car is in pretty good condition, but obviously it has some slight minor issues, like the a few scratches in the paint and slight crack in the leather, nothing too serious, and isn’t the main concern. but since this is a second hand car I don’t really know how the previous owner maintained it, it was partially agency maintained.

Now I would like some advice on how to continue maintaining the car and taking care of it for the coming future to avoid any or rather as many problems as I can.
A friend of mine told me it’s best to try and flush all oils, change any rubber parts, all spark plugs, engine mounts, brake pads, tires, etc. so that the car is like “fresh” again.
Which honestly makes sense, but right now I wouldn’t be able to “afford” doing all of it.

So my main question to you guys would be what are the main/first things I should check and change or even do.
and also where to do it, because a lot of the garages aren’t very trust worthy and don’t usually do the best job.
And also any places for the interior and paint (not the priority, but just for future when I have extra cash to fix)
And also if there’s any stuff I can do on my own to keep the car well.

And also another thing is the mileage of it, I’m getting anywhere from 16-22l/100km, depending on how long I’m in traffic for. I know it’s a v6 and all but is that normal or could it be made better, obviously other then driving style 🫣

Thank you in advance, all help and advice is appreciated.
Thank you.