The extra-strong springs on the Police model are:
The magazine spring.
The shell lifter "carrier dog" spring.
The Trigger/sear spring.
Note that the extra strong magazine spring is used ONLY on the 4 shot standard length magazines.
Remington uses the same springs in guns equipped with a magazine extension, so you won't need to buy a different magazine spring for a Magnum Marine.
I'd suggest not installing the trigger/sear spring, since this was intended to make the trigger pull a little heaver to prevent accidents.
Contrary to popular belief, Remington doesn't use stronger shell releases in the Police guns.
The Police and Wingmaster use a milled extractor.
The Police does use a compressed powdered aluminum trigger plate, but the plastic Express plate you get on the Magnum Marine is just as durable, has no finish to wear off, and will bend, flex, and return to shape if bumped where the aluminum will stay bent or break.
So, about all you need to do to upgrade as much as possible to the Police model is to install the carrier dog spring.
Current 870's are coming from the factory WITHOUT the locking safety button, so try to make sure you get a new, current model, or you'll have to replace the safety.