Go visit the TMC forums and search to your heart's desire. Here's my take:

Paint protection:

For new cars, the debate is always around whether you should get a ceramic coat or clear bra/paint protection film (i.e. ppf). But seeing as it's used, you should see if your car already has anything on it first. You obviously don't want to apply any of these options if your paint is already damaged (i.e. nick/pock marks on front of car), as you'd want to get it fixed up first. I did PPF on most of the front of my '17 X90D for $900, but that was right after I picked it up new.

Color Change:

If you're looking to wrap the whole car, that will be more expensive than just protecting the paint. Could get up to $5k or more for a Model X. You'll want to talk to local shops about their options & costs. Or find a TMC or facebook Tesla group that will recommend shops in your area. You could also call your closet Tesla service center to see if they know of any shops that they recommend nearby. Keep in mind there are no set prices for any of this. It will depend on the shop's prices, your car and it's condition.

Tint:

Most Tesla's come with the rear windows tinted. If your used car's fronts are not, you may opt to get them tinted by a local shop. I had a local place do my front 2 to match the rear windows using quality 3M tint for ~$100 in a couple hours. So this one's pretty easy/cheap. Some people get additional tinting on their windshields/back window/roof windows, but the odd shapes will add to the price, and I didn't see the need. Nor did I want to mess with visibility out of the front or rear of the car.

Wheels:

You can get your wheels dipped by local shops. Usually it's people getting silver wheels done in something darker, like black. These shops can usually repair curb rash as well if you have that on any of your wheels. Or you can buy nicer/bigger wheels from Tesla's on-line shop and they will install at your local service center (maybe mobile service?). Just keep in mind that the better looking 22 inch wheels will reduce your range a bit, go flat more easily and cost more to put tires on vs. the smaller 20-inch wheels.

Chrome:

Chrome delete is a popular option. Usually those local shops that do paint protection or wraps can do this as well. Basically applying a black film over all your chrome trim to get a look many prefer.

Interior:

I bought some all weather floor mats that turned out to be really great for much cheaper than what Tesla was asking. Just remember to search Amazon for the Model X with the right number of seats.

Otherwise you can just browse the Tesla shop or Amazon for any Model X accessories that interest you.

Edit: if your car sill has the old MCU, the best upgrade you can make is getting the new MCU installed by Tesla. It's $2k for the upgrade or $1.5k if you don't care to keep FM radio.