Advice on Purchase of Used '17 GMC Sierra

I've never bought a used vehicle b4 but am looking @ a ''17 Sierra with a 4.3L V6 engine & 6-spd automatic transmission. The truck cosmetically is top shelf for a '17 both inside & out. 165,000 miles. CarFax says that's 27,000 per yr. I figure those are highway miles since that's 100+ miles x 5 days p wk. It was a 1-owner. Auto check lists it as a leased vehicle. The local power company had the truck.
The seller told me up front his dealership was taking the vehicle off the market because they'd already put a buncha $ into it only to discover a bad torque converter, but since I called b4 they took it down they'd put in a new tc & sell it to me for the asking price. They'll also let me buy it "as-is" for $1500 less. If I don't buy it it's being sold to a wholesaler. Local shops are telling me the $1500 is the right price for the tc.
The price is great. The vehicle looks great: new tires, brake rotors & a buncha other stuff. I've not seen anything as good cosmetically out there for the $.
Local shops are all telling me they'll definitely do a tc & pump replacement & a tranny rebuild partly due to the miles for $4500-$5500 total.
The dealer is offering a 30-day 50/50 warranty. I'd be out the $1500 for the tc, but @ 50/50 only another $1500-$2000 for the tranny rebuild with the dealer picking up the other half. .. sounds like a great deal for me as long as the dealer would hold up his end of the deal & pay his half. And from what I've seen even if the dealer refuses to pay his half & no other issues remain it'd still be a decent deal for me.
But the dealer won't let the truck leave the lot to be inspected by another shop because he says he doesn't want anyone else screwing with it. I drove it & it runs great. .. engine compartment & everything I could view looks great.
All these sites online say I should have another shop look over the truck b4 a purchase but that's not going to happen per the salesman. Would it be foolish to buy the truck?