You can buy my Sankey Wide-Track (has tailgate) for £500.
Wheteher you choose to or not though, you can't really beat starting with a Sankey wide-track as a basis... it was purpose-designed as an off-road trailer, has NATO jaw so no problem towing at strange angles off road, has more than adequate ground clearance, is braked, is easily converted to 7-pin trailer socket wiring (mine already done), all the brakes, lights and everything else is road legal too. Plus it's built to withstand squaddies.. so is very tough.
Finding a wide track isn't easy though - they are rare, and finding a non-rusty one is even trickier. Hence mine at £500 - it's in perfect condition.