Also, where does the value of 18-20% off MSRP of a 2019 vehicle in 2020 come from? Just curious as to how someone came up with this number to shoot for.
Sure, this is purely a data-driven number. Let me explain further...
One advantage around buying a late model vehicle is the fact that there is data out there already around resale values, which means you'll know what the fair value of the vehicle truly is. There are many sites out there that provide resale values, such as Kelly Blue Book, Edmunds, etc.
With this in mind, you should "run the numbers" to find the resale value for the 4Runner that you want to buy, with only say 50 miles on it. Run the numbers for both Private Party resale and Trade In value, in "Very Good" condition. As all dealers will tell you, absolutely no vehicle is in "Excellent" condition, so you should not select "Excellent" condition. I usually like to be right around Private Party resale, in "Very Good" condition. This way, the moment you drive it off the lot, you should have pretty good confidence that in the event of an emergency, you could probably sell your 4Runner the very next day, and get most, if not all, of your money back
Bottom line, for me, it's all about the data and resale value, so that I know with a high level of confidence that I bought a vehicle for truly what the market says it worth. This strategy has paid off pretty well for me over the years, and I've typically been able to be around $200-$300 in monthly depreciation for all my vehicles (I've owned over 40 vehicles, all bought brand new, sell within 1 year of purchase, never put one red cent into them for maintenance, tires, brakes, etc.), from the moment I buy them, to the moment I sell them (I never trade-in, and always sell Private Party), and I typically never finance (unless it helps to get a better deal, and then I pay it off immediately, within 1 month). I'm comfortable driving a brand new $50k vehicle for only $200-$300 a month each year

. For me, it's a bit of a hobby.
I know some people may find this as a waste of money, and my reply would be this: Think about the last time you bought and sold a vehicle, and calculate the difference between your buy price and sell price (or trade-in, if you traded in), and also add any money you spent on maintenance, and then divide that number by the number of months you owned the vehicle. If I had to guess, that number would be more than $200-$300 per month.
Also, last point, in the state of Missouri, we pay taxes only on the difference between the new car price, and the price of the vehicle sold, and we do not need to buy and trade-in with a dealer to get this tax advantage, since we pay our taxes at the DMV when we title our vehicles here. So, for example, say I bought a vehicle for $50k, and sold my other vehicle for $45k, I would pay taxes only on $5k, and NOT the full $50k. In this example, my tax bill would be only $400 (8% of $5k). This works for Private Party sales, too,
Hope this helps!