There are no night trains for most of these distances, so your trip doesn't work.
(day 1)Tour Krakow and Auswitz, than at night take a train (instead of an hotel) to Vienna.
No night train, but a through coach (routed car). Special reservation required. Price should be around 100 EUR including the reservation fees for a couchette.
(day 2) Tour Vienna, and than at night take a train to Salzburg.
No night train from Vienna to Salzburg, because it is only 4 hours away with the EC train.
(day 3) tour Salzburg, and than at night take a train to Venice.
There is a night train, it leaves Salzburg at 01:34 and arrives in Venice St. Lucia at 08:34. It is the EN 237. Not a good connection if you ask me.
(day 4) tour Venice, and take a train at night to Florence.
No night train from Venice to Florence because travelling time by fast Eurostar train is only 2 hours and 4 hours by slow regional train. There is a night train to Rome though.
(day 5) tour Florence, and at night take a train to Budapest (I have to be that day in Budapest, so I can't go after Vienna to Budapest).
No night train from Florence to Budapest AFAIK. There is a night train from Venice though, the EN 241. It arrives in Budapest at 11 am (if it isn't delayed which often happens with long-distance trains). If this is soon enough for you...
Your trip would work better if you took the night train connection from Salzburg to Florence via Villach. (Depart Salzburg 21:10 with the OIC 596 to Villach and take EN 235 at 00:03 from Villach to Florence.) Then you could go from Florence to Venice and take the night train from Venice to Budapest. Or maybe skip Florence altogether, instead of spending time on the train you could use it better for sightseeing in Venice.
[ Edit: Edited on 16-Jun-2009, at 16:13 by t_maia ]