To cover off the fuel economy bit, you'll be wanting the 8v DIGIFANT which will get at least 40mpg, prob even better if you take it easy. Note that some 8v's are on K-Jet injection which is (IIRC) a bit faster, but worse on MPG.
As for reliability etc. I do 150 miles a week in a 16v and would thoroughly recommend it. It also sometimes does 300+ miles in a single weekend if I visit my folks in Derbyshire, and handles the journey admirably, to the point where I love the long drive! When I first got my 16v I used to stop half-way to Derbyshire to "give it a rest", but now I don't even bother doing that.
Things frequently do go wrong with an 17 year old car, ranging from silly little things like an electric window motor giving up, to big things like the head gasket. But... Even brand new cars go wrong and let you down, and to be honest I know of LOADS of people that own modern cars that have all sorts of weird electrical gremlins (because modern cars are so complicated) that even the dealership can't be bothered to trace/fix the problem.
I miss aircon but then you'll get a sunroof and windows, and hey, you'll be saving the planet.
In summary...
1) More things will go wrong, but there easier/cheaper to fix. Plus without depreciation, you'll have more money to spend on garage bills.
2) I wouldn't buy one unless you intend to do SOME of the work yourself, otherwise it would get expensive.
3) Buy one as a second car for a while and iron out the kinks/see if you like it before getting rid of your other car and being dependant on it.