Hi Faisal,
Finding a place under 100EUR will be tough if Venice. We stayed at the B&B Corte Campana (www.cortecampana.com) which is recommended in RS Italy book. It was 125EUR a night and included breakfast. It was very close to San Marco Square (5min Walk). Riccardo the owner is very helpful and accommodating. He was always willing to give suggestions on places to eat and things to see. He even invited us to a local bar with some of his friends to watch a soccer tournament. Very friendly guy. I would definitely go back there.
As for restaurants to eat, we went to Trattoria da Giorgio ai Greci (which is in RS book also) and it was one of the best places we ate in all of Italy. They have Canal side dining in a quiet area off a back canal. It was so great. Took a little while to find but well worth it.
In regards to time, I would recommend staying at least 2 nights. That's what we did and we only had enough time to really see Venice (didn't have lot of time to go to Murano or the other islands).
Have a great Trip! Venice is AMAZING! :-)