You won’t have any difficulty actually operating the car for the trip. The brakes, turn signals, etc are the same as all other cars. Read about the wipers if you expect rain. If you plan to use autopilot, read about setting the speed before setting off. 800 miles will entail use of supechargers on the way so you’ll need to know how to plug it in and then unplug it. It’s easy but you want to know before having to do it. Also it’s worthwhile to know how to display the manual. At your first supercharger stop, you can look up whatever you need.
Sometimes it’ll take a day or so for new software options to be installed. With new cars, the music streaming takes a day. If your car didn’t have autopilot installed before and you have it, then it might not work right away.
Run the VIN through Carfax. If there’s an insurance claim it’ll hopefully show up. Not always, though. Keep in mind they seem to report about everyting as a collision. It may or may not be. Tesla won’t tell you the history of the car so it’s up to you to find out all you can.
Look at the car carefully before signing the papers. Once you sign, it’s yours, warts and all. The time to look the car over and discover any issues is before you sign, not after you get it home. Most people take great care of the cars, not everyone, though. Look the car over in good light, not in the rain. If the car is wet, ask them to dry it. You cannot see paint issues with a wet car. Look at the interior in detail, if paint is important to you look that over as well, look for door dings. Check all the sheet metal gaps, that can indicate body work. Check bumper gaps. Look under the front, these cars are low, it’s easy to scrape the bottom of the front bumper on a concrete parking stop. Check hood fit, glass fit, check for glass cracks, windshield chips, wheels for curb rash, sidewall rips in tires, etc. Check for general cleanliness. If the previous owner kept it scrupulously clean, he probably took the same care of the car. If it’s not clean, well... Look to see if there’s XPel or a wrap on the front bumper and hood. If so, that owner really cared. Check tires for tread. It’s $1000 or so for a set. If tires are worn, look at the wear pattern. Make sure you get all the parts to the charger. Look at the 17” screen, the edges. Depending on the age of the car there can be discoloration.
Smell the car. If it’s a smokers car, you’ll smell that for as long as you own the car. No one will ever get that smell out. Ever.
Read through this site for issues. Keep in mind people that find things tend to be vocal. Just because someone else had an issue doesn’t mean you will have a similar issue, most of our cars are absolutely perfect. But it will alert you to things to check. I’d make a checklist. It’s easy to leave things out if you don’t have a list.
Finally don’t let anyone rush you. Getting the car papers signed quickly benefits the seller, never the buyer. If the delivery person seems pushed for time, that’s his problem, it’s certainly not yours. Tesla has a lot of cars. There might be another one that’s a better fit for you. Or this could be the one. You should love the car, you should be absolutely sure this particular one’s for you.
It’s a used car, expect some minor issues but at the least, know before signing what all those issues are. Any issues are likely to be cosmetic. Cosmetic flaws are generally fixable. Not all of them, though. Tesla no longer fixes the flaws, if you find ones that you’ll want to fix, get an idea of how much it’ll cost. If parts need replacement, well, Tesla parts aren’t quick nor are they cheap and there are no secondary suppliers.
You’ll be 800 miles from home. Check for local hotels, if there are issues that you’d like to think about before signing, you’ll have time to think about it. Having a place to spend the night gives you an option and avoids feelings of pressure to sign right then.
The car itself should be good for as long a trip as you want to take.
Welcome aboard, these are absolutely great cars.