I'm hoping to finally pull the trigger on a twin towards the end of the year and have been looking into the same sorts of things.
I found a neat comparison site that also includes a loan calculator
http://www.ratecity.com.au/car-loans just click the orange "find a loan" button for a more detailed search. This is helpful in getting a ball park idea of the likely repayments.
You will want to speak with an advisor or two and work out a list of questions you want addressed. Doing so via e-mail is not a bad idea so you have something in writing.
If seeking a mods friendly insurer I'd recommend looking at
http://www.shannons.com.au/
A family member that had an s15, that they moded over the years, and never had any problems with Shannons. Their whole angle is geared towards the enthusiast market and (more often than not) includes moding.