With air and lodgings covered, your budget is just fine. The nice thing about Rome is that there are so many things to do and see which cost little or nothing at all! You'll have enough to cover all the usual attractions which have entry fees, though.
Getting from/to the airport (which I'll assume is Fiumicino):
The fastest way is the Leonardo Express train, which is 14€ pp and take about 1/2 an hour. If you're really wanting to pinch pennies, you can also take a shuttle bus; Terravision or SIT are good options. Either cost around 5€ - 6€ pp, take about an hour, and you can buy your tickets at the airport. Depending on where your accommodations are, it's possible you can walk or take the metro (single-ride BIT tickets: 1.5 €) to it from the bus stop.
http://www.adr.it/en/web/aeroporti-di-roma-en-/pax-fco-bus
As noted, the Borghese requires advance reservations/tickets so do that before you leave home. To skip long lines at the Vatican Museums, do the same; here are the links for both ticketing sites:
http://www.museivaticani.va/content/museivaticani/en.html/
http://www.tosc.it/tickets.htm?affiliate=T2C&sort_by=event_datum&sort_direction=asc&fun=erdetail&doc=erdetaila&erid=912678&language=en
I'd normally advise pre-purchasing for the Colosseum/Forum/Palatine as well but during low season, you can also go to the Palatine FIRST (ticket lines are shortest there) to get them. Once you have that ticket, you can skip the long ticket line (but not security checks) at the Colosseum. A single ticket is good for all three attractions but the Palatine and Forum are connected and must be visited at the same time, without leaving the secured perimeter.
You're going to have a wonderful time! :O)