What us the best approach for importing historical transactions into NetSuite? The most important for the particular prospective client we are talking to will be customer invoices.

What is the industry and what is the legacy system? I can potentially help you. I have a proprietary tool that migrates legacy data into NetSuite. I also have an article on my website discussing how to import historical sales. Happy to chat more if it would be helpful for you and your client.

when you upload transactional data sum of your balances on invoice must equal their trial balance for the same period. If you must upload paid invoices you also must upload payments. Not common but happens.

I've seen historical invoices uploaded as Cash Sales to a clearing account and then the clearing balance reversed

There are many parameters to take into account in a financial migration. It all depends on the degree of information from your old system that you need on your Netsuite. You must import GL, open transactions, interco transactions, deferred expense and revenue , if you are doing revenue recognition , fixed asset