I'd leave the bridge alone unless it isn't functioning correctly, way too many people waste money on bridges. If you must change the bridge, get a cheap high-mass bridge from GFS, ebay or somewhere else. IMO the bridge on 99% of basses (even most lowend) are perfectly fine as long as they work correctly.
Knobs are fair game, whatever makes the bass more aesthetically pleasing to you and helps personalize the instrument will encourage you to play it more.
As for pickups, this is also fair game, but to be honest, what you've got already has the potential for the sound you are after. If you're after a 90's punk tone (think Matt Freeman P-bass type of tone in Operation Ivy) , choose a bright set of roundwound strings and turn up your tone and play with a pick. If you really must have something more punk oriented, I'd go with either a Seymour duncan Quarter Pounder (My main bass has one and I play punk from the 70's right up to modern punk) or a Fender original '62 pickup. You may doubt the use of the original '62, but most punk players used whatever they had on hand, and 9 chances out of ten if you like the vintage punk p-bass tone, it was the stock Fender pickup.
The Extender would be a cool upgrade, but only really useful if you need to rapidly droptune in frequent increments. I find they are fairly expensive, considering I don't drop tune very often. I'd personally buy a good pedal tuner instead of an extender, but that's my opinion and ymmv.
Edit: As most people above me have already said, your best purchases for this bass would probably be new strings and a pro setup.