You can do this while maintaining a positive balance. Here's how:

Sell all improvements in your three starting cities (raises balance to about 27000)
Disband all units but one general in Mediolanum and disband the 3-unit army in the south. Disband all cavalry.

Leave 1 Lugoae in Bibracte so you can keep taxes higher. Move everything else to your ship in Cenabum. You will end up with one full shipload: 4 generals, 4 Botroas, 7 Lugoae, 5 slingers.
Travel to Ivernum and lay siege. Your slingers will kill off about 40% of the enemy before you enter the town. Disband your ship.
Leave a general behind and besiege Emain-Macha.
Leave a general behind and besiege Attuaca.
Besiege Caern-Brigantae (this fight will be close but you will win)
At this point you will have about 5000 in your treasury and about 800 men in your army. Establish trade with the Sweboz.

Now build up your farming and trade. Build a blacksmith and a field of games in all your cities.
Once you have a healthy cash balance, establish type 4 governments in all cities and crank out lots of cheap spear levies in Ireland and Caledonian Skirmishers in Attuaca. Troops from Emain-Macha will have +3 experience.
Sooner or later the Casse will break the alliance. Wipe them out, and continue to build units until you are just breaking even. You should have at least two full stacks.
Invade Gaul and overrun the Arverni. They will have built lots of improvements for you.
Once you have all of Gaul backed up by Britain as a finamcial powerhouse, Rome is no match for you.