A low mileage used car can often offer excellent value. (I bought mine used.) But to get that deal you may have to be patient. Not all used cars are great values. You must find a dealer that is willing to accept minimal profit on the car.
You didn’t mention this, but will you need a loan? That may change your strategy. Interest rates on used cars are significantly higher than on new cars. And if the dealer will only offer a four-year loan on a used car compared to a five-year loan on new car, the payments on a less expensive used car can be higher than on the more expensive new car.
One other thought. You implied you will sell your current car privately. How will you time this? Will you wait to first sell the car and then hope to find a used GTI quickly? It might take many months to sell your car privately for your price. It might also take you weeks or months to find a used one that is a great value. That could become frustrating. Remember, trading in a car will save you on sales tax.
Several things must fall into place correctly for a used car to be the best value. If the used car isn’t a great value, I’d suggest getting a new one (at dealer cost.) I will argue the warranty is an issue. The powertrain stuff rarely fails on a GTI. It’s generally the smaller things that are not covered by the powertrain warranty. Once the 3/36 B2B coverage expires, you really have a lot of exposure. Getting a new car gives you a full three years or 36,000 miles of full coverage.
New and used cars have their advantages. The hard question to answer is at what price does a used car become that great value. If the car was traded-in early in its life, why? Is it one of those rare lemons? Food for thought.