Colin,
I am in a sub aggregation group - mainly for PD additional hours, but I have not heard of any of them talking about order of lenders. All are transactional brokers, market generally first home owners, they all say they do investment properties but usually just one IP, they all tend to use just one or two lenders, either because that is who they used to work for and understand the processes or are a volume rewarded broker with the lender.
I worked in out as I was an investor before I started in this field as a mortgage broker and soon realised to build a property portfolio, first you had to qualify to borrow and you then needed to purchase an IP with the characteristics that would enable you to borrow again. I learnt from experience the unmentioned credit limits applied by lenders to individual borrowers.
In addition to the questions you suggest to ask, also find out how many IP's they have and how are they held. This will give you an idea of how serious they are in this business or that they are just a disinterested 'banker' giving lip service. My view is that to build a successful property portfolio, you need to understand both sides of the transaction, first the asset itself, what type of IP in relation to purchase price (relates to own funds to settle and what you can borrow), to capital growth prospects (for future revalue and refinance strategy) and to rent yield (serviceability) and secondly the finance equation, who to use to borrow, what LVR works best and which lender in what order.