This a very late model vehicle. So it hasn't been around that long for common issues to be known by the wider public who are not 435i owners.
Here's what I would do:
1) I would first pull the maintenance history with BMW by using the VIN and see what work has been done while the vehicle was on plan. Big ticket replacements like engine / gearbox under plan will indicate poor reliability, but this could also have been a Monday (lemon) from the factory.
2) I would check the VIN with
https://www.wheelindex.co.za/vehicle-history-check to see if the car has ever been in an accident, current finance status and theft status. Accident data can also be found as a comment on the motorplan, it wont have details - so you will need to do more research if there are accident comments
3) Engine issues. From my research models before 2015 used the N55 engine and thereafter the new B58 engine was used. I would research these engines, you will likely find many posts regarding the N55 engine on this forum as many owners have vehicles fitted with them, many have also done extensive maintenance.
3) Tyre wear on the inside wall of the tyre I would look out for, replacing run-flat tyres are very expensive!
4) If the vehicle is out of motorplan I would buy an extended warranty, just for peace of mind in case something goes pop.
5) Get a multipoint check done for your own sanity - BMW can do this for you have and it will highlight common issue. Like wear and tear items. This should give an idea what the car needs immediately.
Hope you find these helpful.