What should I check before buying a '16 TRD Off-Road Taco?

I’m looking to make an offer on a 2016 TRD Off-Road Tacoma with 143k miles. I don’t fully trust myself to assess a used truck, but the dealer is allowing me to bring it to a mechanic to check it out. Any advice on key points to focus on? I live in the northeast, so a clean undercarriage is a priority. What else should I be looking at? Thanks in advance!

Make sure it goes into 4WD and check the lockers if the truck has them. Also, inspect all the fluids.

I have a 2016 Tacoma with similar mileage. Is it stock?