Where to start.
Bike that was sitting and not running when parked.
You're off to a good start.
With fuel tank apart I'd replace filter and all in tank lines with the correct OEM or comparable rated.
Quick disconnects on gas lines a weak point, replace.
But before anything like that I might jump into the brakes.
A bad ABS is $$$$$, you can bypass it if needed.
Complete flush and cleaning.
Final drive and trans pretty straightforward to drain refill.
Check FAQ's here and here.
There are plenty of online sources.
The second battery is not necessary but can be used on an ex P bike.
If the tires were ?? years old when parked, check date of manufacture.
Often the police wiring is circuitously circumspect so post specific to that model and owners here will provide great links.
I'd Start a new thread wrt that.