1st- trust me. you have NOT thought of everything. It's just not possible. But not to worry! That is all part of the adventure
If you go to Rome, steer clear of the M&J Place hostel near Termini station. Why? Well let's just say that on one particularly hot summer night some other kids and I kept ourselves entertained by throwing popcorn to the rats on the ground below our eating area!!
I've heard a lot of bad things about the Three Ducks hostel in Paris. I stayed at Culaincort hotel in Montmartre. It's one of the nicer areas of Paris and I'd recommend checking it out.
I stayed with family a couple of times while I was in Europe. On one particular occassion I stayed with an aunt in Turin, italy. Her place was absolutely, indescribably gorgeous! Blue marble floors! Bath towels the size of bed sheets! and all I could think was "god, someone get me the hell out of here!" I just didn't feel the freedom I felt staying at hostels. After one day I just left the city altogether.
I found it impossible to budget by day (but that's just me) so I just budgeted about $1000 a month and usually came up under-budget.
And props to the Generator!