[vded]

I have my 116i N13 remapped a year now. Power gains, 225hp/32kg on 1.0-1.1 bar.

I have the stock air filter. No problem.
I haven't changed the spark plugs. No problem.
Intercooler I have fitted the OEM from N20 engine (125i model). Here in Greece we have plenty sunshine and high temperatures 8 months a year. No need for a bigger or more expensive intercooler.
The gains for changing the exhaust are minimal, not worthy in my opinion. I have fitted only the M135i backbox for the double exhaust looks.
GFB DV+ installed back then. Makes good work on MT cars.
Catalyst change to a new one doesn't worth. It gives only around 5 more hp and plenty of unpleasant smell when you start the car.
No oil exchanger / oil cooler needed.