You're right in saying rusty parts need to be treated first, just spraying over them with underseal is fairly pointless.
If you treat them first with a good rust converter, I have found Blit Hamber products to be good for that http://www.bilthamber.com/corrosion-treatments.
Then for sealing the metal POR-15 is excellent.
Dinitrol so a really good cavity wax for the box sections and hidden areas and I have found Terrotex HV350 good for converting steel suspension parts.
Its not expensive if you do it yourself just time consuming to do properly but very worthwhile as no 350Z came with proper protection, UK or Import.
For the cavity wax you will need an air compressor and underseal gun, some good products on here http://www.rust.co.uk/