Questions before buying unlocked phone

Hi,

I'm looking into buying a Z5 Compact unlocked since I don't want something 5 inches or bigger.

I'm hoping to not install custom roms, at least not until Sony stops supporting it.

I've got a few questions :
Will I be able to get the stock rom updates from Sony or manually install them (without unlocking the bootloader)? My carrier doesn't sell the Z5 or Z5 Compact so I doubt I will receive updates from it.
I have read a few threads that makes me worried about the phone. People seems to say it's sluggish and the camera app takes a while to start. Is it working better on the latest stock rom?
Is the stock rom good or should I really look into flashing a custom rom?

Would you guys recommend another Android phone? I would like a compact phone that can last me for a good 3 years, maybe a little more. Right now I own a Nexus 4 that is a little over 3 years old and while it still working rather well it is starting to show signs of its age (like the battery that doesn't last for very long anymore and the phone sometimes struggle running the latest games...)

Thanks