Like you, I wanted a good reliable 400 and I spent a long time researching into which car to have, using as many sources as i could. I came to the conclusion that the Series 3 (95-7) fitted the bill best, taking cost of purchase and specification into parallel consideration. The 95 had lots of improvements over the ser 2; such as more power, less fuel consumption, more leg and shoulder room, bigger boot,....even thicker glass. Taking into account that you have to relate to market value, then your £10k should buy you a low mileage ser 3 with full serv.history Mine was 50k miles, at £10k, and that was 18 months ago. Just be careful. Often garages get them in p'ex for a newer car from the proverbial 'careful owner' and just want shut of them, as big second hand cars aren't too much in demand. Try and read the 'Car' reviews of the 400 by Setright, they'll reassure you.(I can locate the details if asked.) Don't be put off by the odd scare story on leaks or whatever. These are legendary for reliability, statistically.
You COULD poss get a (high mileage) ser 4, but the laws of physics still apply. Things DO wear out with miles driven, so the less miles seems a number one concern?
It's the best car I've owned (my last was a BMW 850, for comparison. Fast, but everything cost dosh too often!) Great to cruise in, yet fast as hell when needed, comfortable (I'm a Citroen suspension long time fan! so, that's praise!) in fact, ...you'll love it when you finally get one!