As a fellow grey beard (unfit, overweight, one leg), here are some thoughts:
Price: maybe £200, but a) these are not good times to sell things and
who wants a ten-year-old mountain bike when for the same money you could buy a two-year-old model with more up-to-date features? Put it up for £100 and get a quick sale, accept £80, and think yourself fortunate.
Suitability: unless you're off across country, through woods and up and down hills, then FORGET IT. Mountain bikes are heavy, have enormous tractor-type tyres, and take probably 25% more effort to ride than a modern 'hybrid' or commuter bike.
After 40 years off, I restarted last October. My first rides were of about 400 yards - it is HARD work to start with. Four months on, it is a bit easier, and I am doing 4 or 5 miles, including horrible hills. Unless you are superfit just accept the limitations of age and rustiness. You will make progress, but it might be slower than you'd like!
And until you decide that you're going to spend every minute of your marvellous free time riding your bike, don't bother paying hundreds and hundreds of pounds on some glossy machine. For £100-£200 you'll get a perfectly adequate second-hand bike from a bike shop which will do you fine, otherwise you'll have ANOTHER expensive bike sitting in the shed doing nothing....
Good luck.
Allen.