Dear thommsf
The train to Stansted Airport, the "Stansted Express", leaves from London Liverpool Street (not Liverpool) station every 15 minutes. It takes 45 minutes for the 37-mile journey. It stops in the suburbs at Tottenham Hale where you can transfer from the Victoria Line underground, as noted previously, but from central London I would take the Underground (or a cab) to Liverpool Street, one of London's major stations, where I could get in the train at the start of its journey, have my choice of seat (standing from Tottenham to Stansted is no joke), and probably find things easier.
Ryanair is an OK airline, certainly for the money, which is often a fraction of the so-called "full-service" airlines in Europe. The aircraft on the Italy routes are usually the new 737-800s; the older 737-200s are more prevalent on the routes out of their Dublin hub. I find their punctuality as good as the major airlines too, despite their short turnarounds, because they don't spend half their day stacked waiting for a landing slot at Heathrow.
There is no advance seat selection. Try for the emergency exit rows in the centre of the aircraft, for more legroom - many passengers shy away from them because of their yellow headrests which make infrequent flyers (Ryanair has more than its fair share) think they are reserved or something).
Eat before you go, sure. Plenty of opportunities at Stansted, both in the terminal and at the remote gates. There are drinks and sandwiches sold on board the aircraft but they often don't have enough, particularly when I'm hungry! (service starts at the front).On one recent trip there was no food, because "They forgot to load it at Stansted". One hopes a more professional approach is taken with the fuel.
The only currencies you need nowadays are Euros in most of Europe, and Pounds in Britain. Being an Irish airline, the latter is known as "Sterling" on Ryanair (not an expression often used in Britain, but we know what they mean - they even write it "STG"). Both are accepted equally on Ryanair. If they say they will be back with change "later", in my experience they invariably do so, so don't worry.
Selling food on an aircraft may be novel, but from Europe we are similarly surprised within the US on the major carriers to be actually charged for liquor. By the way, steer clear of GBP 50 and GBP 100 bills in Britain any time you change money as they are really not in common use; GBP 10 and 20 are the ones to have. If you take your money out of an ATM this is all you will get anyway.
I would be quite happy about winging it round Italy at this time of year. In tourist terms Italy is more markedly seasonal than most Mediterranean countries; June to September are the peak, so you should find plenty of space (except if you are planning to be in Rome over Easter, in which case reserve ahead).