The earliest you can travel to Venice is after sequence 6 after you have completed the bleeding effect.
Paintings are sold at Art Merchants denoted by the Paint Brush icon on your map (You will need to synchronize with the view points in Venice to find the Art Merchant), the cheapest painting you can buy in Venice is St-Jerome for 53 Florins
and the cheapest painting you can buy in Florence is Baptism Of Christ for 280 Florins
. So you will need at least 333Florins to get this which isn't a lot at all and the paintings increase the value of the Villa which makes you more income anyway.
The fastest way to get between the two cities is to use the Fast Travel stations.
Note: The achievement will not pop until you exit the store
Edit: According to LuckyConquerer2 there is a cheaper painting in Florence called 'Portrait of a Musician' 20 Florins.