Totally understand the finance thing and good for you for understanding it and not just slapping down a credit card and running up some debt.
There are things you can do to achieve a fast position on a road bike, especially if it has a round seat post. You can either rotate it 180* or get something like the Redshift seatpost which will get you in a more forward position.
I used to race tri’s on a Cervello S2 with the reversible seatpost and dedicated TT bars…and I would argue that I was not giving much up to the TT bikes in time (in fact, I set the fastest bike split in a Sprint on that bike…not just my AG, but the whole race).
It is also worth noting that if you have a background in both swimming and running, then you don’t need to be as concerned with your bike leg as many others. Tri is 3 disciplines, bundled together. If you can excel at 2 of them ( especially the run) you will do well in races.