Congrats on your new PPV. I recently bought a retired Florida Highway Patrol 2012 PPV and had to do some interior modifications (Steering wheel with radio controls swap, add center console seat, add DIC switch, add 3rd row seat). I posted updates on these in my thread if you want to view any of my write ups.
https://www.tahoeyukonforum.com/threads/2012-tahoe-ppv-new-owner-here.117090/
Just recently got it back after getting new paint job and it looks brand new.
The 3rd row seat is pretty much like what everyone said, you can buy the bolts and seats used online. I bought my bolts ($43.00+) and seats ($630+ Free shipping) via Ebay. Some seats can get pricey, but you just have to shop for the best price and make sure the seller (on Ebay) has a long history of very good feedback and near 100%.
Regarding the tires. My PPV came with Goodyear RSAs which is what the FDLEF typically use on all their service vehicles. The tires look fairly new and seemed to look like they had a lot of good tire tread, but they were terrible. Had so much vibration at highway speeds so I took it to my local Discount Tire and had them re-balanced, but that didn't help as the vibration was still there. I don't know if it was because it was sitting at the auction lot for about month or so, so I suspect the tires developed a flat spot, therefore I took it back to the tire store to replace them with a new set. I was going to get the Kumho Crugen HP71s, but the tire shop said they no longer stocked them because they were discontinued so went with the BF Goodrich Advantage T/A Sport high performance tires. What a difference that made. The vibration is gone and the ride was a lot smoother cruising at highway speeds. Can't be happier with these tires.
As far as ride handing, yeah my PPV it is a little stiffer than a non-PPV civilian Tahoe, but it doesn't bother me. I'm used to driving sports cars with stiffer suspensions.
As for the push bar and the spotlight, I think they're pretty cool so didn't remove them.
Good luck with your project.