Be sure to FULLY exploit the exploit!
Person 0: Play normally. (+0BC) /* you'd have to be pretty dumb to do this */
Person 1: Naively exploit the debt limit. (+13000BC, lower morale) /* not bad, but you won't get 1st place in the metaverse. */
0) Build a navy of mixed hull sizes. Make the medium hulls as cheap as possible: nothing but 1 of the cheapest gun each.
1) Spend until your treasury is 0. (T=0)
2) Buy tech X from minor race: -2000. (T=-2000)
3) Sell tech X to the 6 races who lack it for 500 each: +3000. (T=1000)
4) Upgrade all (40) medium hulls to the top of the line @400 each: -16000 BC. (T=-15000)
5) Magically generate 13000 credits from nothing. (T=-2000).
6) Suffer morale penalties for a few turns until your debt is gone.
Person 2: Cleverly exploit the debt limit. (+78500BC, no morale penalty) /* for those who want to win */
0) Build a navy of only medium hulls; disband everything else as a waste of upkeep. Make the medium hulls as cheap as possible: nothing but 1 of the cheapest gun each.
1) Buy tech X from minor race: -2000.
2) Buy this bad ship design until your treasury is -500. (T=-500)
3) Upgrade all (200) medium hulls to the top of the line @400 each: -80000 BC. (T=-80500)
4) Magically generate 78500 credits from nothing. (T=-2000).
5) Sell tech X to the 6 races who lack it for 500 each: +3000. (T=1000)
6) Suffer no morale penalties because your treasury has a positive balance.
See how it works? You have to design your entire military strategy around getting the most credits from magic, and your diplomatic policy, too. In other words, timing is crucial - anything you do BEFORE the upgrade is "warshed away by the river", so sin all you want before the upgrade, and save your good deeds for after... and you'll end up with unlimited magical credits, no penalty, and a positive treasury balance!