Newbie- golf 4 gti (AGU)

[JogiahMk4]

Recently got her as my first car ,wanted something reasonably priced ,has a few creature comforts and is reasonably powerful. What are the common things I should be looking out for ? I recently got her serviced and had to change the v belt as that snapped on me after the first day of ownership as the previous owner had it installed incorrectly, other than that she runs perfectly . The only issues I'm noticing is that the average fuel economy reading says 99L/ 100 km , the door locks seem to be sticky, the passenger door requires a gentle tap before it opens and the drivers door needs to be pulled twice to open and lastly the check engine light doesn't seem to ever come on ,even on start up. Any advice? Especially on the check engine light ,im hoping the fuse wasn't pulled prior to me buying the car to hide something.