Just to help
Hi there 1st of all if you're buying a 520 just make the extra jump to the 528i its better MUCH better especially if you're chasing power later on. That being said if you've got the cash to splash the 535 is hard to beat.
As the saying goes if you want a fast car just buy a fast car. It'll cost you more trying to make a slow car fast. And yes the 520i engine and 528i engine is exactly the same N20 engine 2.0L turbo.
Any year after 2014/5 is good, mine came with sunroof, suction doors, adaptive headlights LED, front and back and 6WB instrument cluster. Get a fully loaded car and you wont spend time and money chasing retrofits. Before you spend ANY $ on a car get a pre purchase inspection done, if the dealer doesn't allow for it just walk away, plenty of F10s in the market.
N20 does have the chain guide failure issue as well as the Charge and boost pipe failing and losing all boost check my thread for additional info.
https://www.bmw-sg.com/forums/threads/f10-528i-mods-and-maintenance-pics.87979/
Basically get yourself a OBD reader and PAY for diagnostic app like bimmerlink it'll be a great help down the road. Regular maintenance is key.
If you're thinking of just putting air oil and water into it and not caring about the rest I HIGHLY recommend you look at another brand of car as BMW's do need preventative then regular maintenance especially if you're going to carry it to the full 10 years.
Read up online lists plenty of common issues for the N20 engine get those covered and hope the previous owner had some brains to care for the car when new.
All the best have fun !