Before I buy a 2002 2500 4x4 sub with 225k mileage what would you have inspected? Thanks

 

Welcome to the forum...
there are all kinds of things you could check out...
all door locks work
all windows go up and down
all doors open from inside and outside
rear hatch and glass open from remote and and other push button feature
all of the radio buttons work
all of the dome lights work in their perspective modes
Turn the key to start no crank and check that all the IP lights that should be on do illuminate...
Get a code scanner/reader and check for stored or no codes... if no codes make sure that all emission checks are in a state of ready... if they are not, it could mean the codes were cleared and any problems won't show up until the vehicle is driven for a couple hundred miles ( or just off the lot)

Check for a spare tire, jack and handle
make sure the seat adjusters work
make sure all remotes work
look underneath for signs of leaks or a recently cleaned underside...
those are some things that come to mind off the top of my head...
You should be prepared to put about $2200.00 into a transmission soon if it has not be done already...

 

Went to local chev dealer and asked the same question. For $150 they will do a complete scan and visual inspection of known common issues (transmission slip codes, transfer case . Put it on the hoist and inspect. The lead mechanic will do the inspection and not some oil change guys. Very tempting. I will ask the owner to pay half of the inspection. By having the owner pay half, if there are "known" issues to them they might not be willing to pay $75 if they know it won't pass.

 

I'm not sure what the price was on the OP's prospective vehicle. If I was selling a 12 year old vehicle with 225K miles, I would NOT pay each possible buyer $75 to tell me all the things wrong with it. Both parties should know there will be things wrong with it. If the buyer is serious but concerned, let him pay.