As far as sleeping in a Defender goes, I think you have made the right choice! The 110 hardtop is the most spacious by far! Insulation is essential though. If it doesn't have a headlining then fit one (or use carpet or camping mats etc) otherwise you will wake up in the middle of the night with condensation dripping on your face (don't ask me how I know!!) :wacko:
As has been said, the example on Gumtree does seem expensive. I know it has had some work done it and some goodies fitted, but you should be able to get something a little cheaper and perhaps slightly younger.