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Once you've got your licence any decent sized or brand dealer will let you test drive a bike and they're never fussed about how long you take.
Just go try a load out. Twins, inline 4s faired, dual sports, whatever.
Also remember you can get lightish dual sports that won't be as good as a proper dirt bike, but still work. Just look at videos from the 30s of riders on giant bikes riding dirt tracks.
I'd add that number of owners isn't as much of a thing with bikes as with cars. A car owner putting a couple of k and then selling triggers red flags. For a bike it could just be someones midlife crisis, or like the guy who sold me my bike someone who gets a new bike every year.
The type of bike that would be most logical for my goals this year would be a small, light dirt bike of some kind, but that suffers from the "I live in the middle of London" problem, which means two hours driving out to somewhere that it could be used, and of course two hours driving back home again, which I hate but there's no way round it.