What kind of car do you want? This will partly be impacted by your budget, but you might also have a favourite brand (volkswagen, mercedes, ford). The prices of (second hand) cars depend on the brand, but also on the mileage, year of production and horsepower. Be mindful of cars that have a very high mileage, because some work might need to be done with those before they are safe to drive. If you buy a van with lower mileage and from a more recent year, than you might be able to still sell it for a decent price. Moreover, think about which options that you definitely want in your can and what would be a nice extra. Examples are airbags (just for you or also for passengers), airco, parking sensors, Bluetooth, window heating, etc.
It is also important to look up the regulations in the places you want to visit. If you want to visit mostly cities, they might have Eurozone’s in place. It is good to think about what type of diesel the car uses.
Another point you want to think about are your priorities and expectations of your perfect little home. Do you want to be able to stand up completely or are you fine with a lower roof? Do you want a fixed bed with lots of storage or is a bigger dining area more important to you? Look up the measurements of different brands, some wider, which might mean that you can sleep width was and gain some centimetres for a kitchen or bench. There is no right or wrong, you just have to think about what is crucial for you and what would be deal breakers.
A good idea is to draw up what you want in your van and how much space each thing takes up. This way you know how big your van should be. However, don’t forget that the bigger the van the more stressful it can become later when you are travelling when it comes to finding parking spaces for example. Moreover, I’ve heard stories of police in some countries specifically looking out for white sprinters because those are known to be used as campervans. So, think about that as well.