Hi!
Have a look around for a LHD cali import microbus, og paint, with seats for example, you could find one for 3-4k, that is solid, and virtually 100% rust free, asisde from battery tray maybe - (mine is 100 solid, but needs a battery tray) make you own mark on the interior, Z hinges from ddfrost on here, make your microbus seat a bed, cheap stove, you dont need a sink, use a washing up bowl, cheap coolbox. Sorted. 2nd hand engines are around 300-500 for good runners, you need to start with a solid base, you will struggle to find a uk bus with no rot.
Take a look at Simon Hall, pm him on here, or Tim at Roadhouse Retro for good solid buses. Other dealers sell RHD rust free buses, such as Scott Wilson for example, rust free from SA. LHD tents to be cheaper mind, you will pay a premium for a solid RHD bus, and a SA or OZ one is the way to go if you want a RHD rust free one. But i don't think you will get one for 3-4k
Me? I'd go for a cheap solid rhd microbus deluxe (sunroof) tin top, z hinges, stock engine and go camping in that, in Delta Green, 1969.......Sorted. Though at the time i needed a more camper like pop top westy, though if i was to do it again on a budget...........
Cheers and good luck finding what you want,
Alistair