SuiteTax is the new architecture for processing tax in NetSuite. It does not cost extra and frankly all new builds are with this tech not the legacy stuff. They are supporting legacy tax but not building out anything new. International Tax Reports is not available for SuiteTax so if your client needs to use that solution they have to stay legacy. The better long term choice is to figure out the requirements (reporting) and fill the gap externally with an accounting firm and use SuiteTax as it is the future of the platform. Migrating anything is tough and what little I saw of the migration tool for legacy to SuiteTax would make me nervous.