Stig

All you need is a small two birth caravan. So long as the chassis and electrics are sound and the shell is waterproof, that's all that matters. Make sure it's capable of twin hookup and has leisure batteries in good condition. That way, you can hook it up to the van and the van will charge the batteries on the go. If the batteries are good, they should see you through the night easily. All the rest of the bits and pieces you can sort out as you go.

I had such a caravan and used it for accommodation when I worked away from home. I didn't really care where I stayed so long as I could hook up to mains electrics if I were staying any length of time. I picked up a heap of stuff needed from ebay and such. Cheap as chips. Considering your are only going to use it for trackdays over two or three days, I'd say that was your best way forward.

My caravan was perfectly serviceable but a bit of an eyesore to some more upmarket camping sites and used to get refused entry or I promised I would hide out of the way in a corner of a field or something. It met an end when a van reversed into it and pushed it into a concrete post which caved in both ends of the caravan. But the point was, I got the caravan for very little money indeed and spent a couple of hundred pound to kit it out and that included an awning. It had everything I wanted inside. TV, DVD, Playstation, microwave, cooker, toilet ect...