If the vehicle overheats it goes into "limp home" mode to protect it from more serious damage, so the power loss may not be a problem, a faulty gauge will still trigger it. Slightly low coolant level can cause the gauge to rise, and so triggering "limp home" without the engine actually overheating.
However gauges are there to warn you of serious and potentially expensive problems, so be careful.
Regarding rust, I have no experience of older Discoveries, but quite a lot of the body is not aluminium, although modern steels have far better anti corrosion properties than they used to.
To set the record straight, all Discovery 2s have Traction Control, but only some have a Diff Lock, if you're not planing serious off roading I wouldn't worry too much.
They do not all have air suspension, generally the 7 seaters, an it is only on the back when it is fitted.
Many of them, I believe generally the 5 seaters, have Active Cornering Enhancement (ACE) This can result in expensive problema as it gets older. In my opinion it has more effect off road, as they appear to fit stiffer anti roll bars to compensate on road when it is not fitted.
