Kiwi
I agree with Nev - it's best to get your ABN up and running from the very beginnining. Some companies only deal with registered businesses, so it pays to be registered, and also insured, for the very same reason.
The GST issue is a bit gray - we bought our business in September last year and were told it was likely to bring in $52k p.a., meaning for the first year, we mightn't reach the GST threshold. Thankfully we opted to charge/remit GST from the beginning tho, as we went over the magic $50k and would've had to pay the GST out of our own pocket from what I hear. GST can be paid optionally - most pay it monthly/ quarterly, but it can even be done annually I think. (We pay it monthly, so it doesn't accumulate into too much of a debt.)
If you can set up a special bank account for your business earnings and have a cheque book attached to it, it will be easier to keep the paperwork in order. We have a separate account off our main account without having established a business account
per se, so money is paid to us personally, even tho we also have a *t/a Twin Cities Lawncare* tag-on phrase as well. We have been encouraging most of our non-cash customers to deposit their payments into our bank account directly, adding the invoice number/s for reference, and this saves us having to physically bank their cheque and add extra time to the overall payment process.
All the best for the new venture/s (marriage being the biggest of all)
Christine