We stayed in Hotel Colosseum last year, it was a short stroll from the Colosseum, metro station, Eur 8 in taxi from central station by day (E25 by night - ripped off at 2 am after getting off the Ryanair transfer bus) and has reasonably priced restaurants nearby. Eat slighty off the main throughfares and you won't pay any more than you pay here, maybe even a little less.
When in Vatican square take up an offer of a guided tour - approx E 25 you'll be wasting your time trying to navigate the place without it, look out for a tall 40ish Irish guy with glasses called Michael, from Ennis, he has studied there for years and was fantastic.
You can check wunderground.com for weather histories for clothing but the absolute essential is two really comfy pairs of shoes and I doubt you'll be able for heels as the legs will be walked off you.
We had torrential rain in Sept so the 2 pairs are essential if one gets wet during the day. Everyone else is the in the same boat so dress code is casual. Bring nice tops to wear over a pair of jeans for night time.
There are a couple of Irish pubs within a stroll of our hotel with perfect Guinness - my hubbie was delighted with a decent night cap after a nice meal and a tough day's sightseeing!
You'll have a ball - enjoy.