I'm a bit puzzled by your thinking, the vast majority of campervans and motorhomes are sub 3.5t, for a single person a van such as ours offers loads of room and weight capacity (or is there something your not telling us :raofl: ).
I'll not bother recommending our particular model as you will struggle to find one less than £30,000 , but there's plenty of other older vans that should suit.
Your main choice seems to be whether to have 4x4 or not, if you do decide on 4x4 then as others have said they are rare and you will have to bide your time, it may be that C1 would be handy for a sprinter or daily in 4x4 upped to 4t. If you go for 2wd it's just a case of deciding layout and how much you want to spend, then going out to buy one.