Hi, welcome to the club...
In my experience, Germany has the best pick of the bunch, take a look at mobile.de:
https://suchen.mobile.de/fahrzeuge/...st=true&lang=en&ms=25200;33&sfmr=false&vc=Car
As mentioned above, the good ones will command a higher price, but there will be gems to be found. I would though err on the side of caution in the case of 'cheap'.
I would also focus, touched upon above, on the interior. The van's basic's shell, running gear and engine will be 'easy' to get fixed and parts found, but the camping interior parts are getting harder and harder to find.
The key area's to look for will be in the fridge, water/gas supply/waste all working correctly. If installed, check the night heater functions as it should. These are the main areas of expense if they're not working. The charger unit can be replaced with an upgrade for around £200 and so is not so critical. General condition of the furniture should be checked as replacement parts for these are becoming rare as rocking horse poo.
To save repeating, there are many good posts to check in the T4 section of this forum with lots of detailed info on what to look for and also how to check for 'fake' vans (T4s fitted with an original Westfalia interior - not terrible in itself, but don't over-pay for a non-original - also could indicate a dodgy past...). The subject of what to look for is well documented on here...
I recommend doing your homework, taking your time and holding out for the right one for you. If you have the chance to spend a week or long weekend in Germany (or wherever you find a good choice) to visit as many as you can (we managed to see over 10 in one weekend in the region around Stuttgart for example). You will then be able to build a clear picture of what's possible for your budget and what to look for. Familiarise yourself with how the camping elements work and check everything for working condition in each van you see - if you haven't used one of these vans before, maybe ask an owner if they would mind showing you around, then you won't be fobbed off by a seller if something isn't working as it should. I wouldn't imagine many owners would mind giving a tour of their pride and joy ;-)
One last point for now, if you're planning on importing, be sure to do your homework on the process before heading off, also maybe boris bike could have an effect on that, but who knows...
Good luck!