Timber species

Hi all,

I started out wanting to replace a few rotten boards (pine), then decided to reap out all the decking boards as they all have some signs of rotten. Then decided to replace a few joists (also pine), then decided to replace them all as they are all stuff. Then realised some of the bearers also were rotten. So ending up a big job instead of a weekend replacing a few boards...

Anyway, I just finished the bearers, joists etc. Replaced them with hardwood and bigger ones as I want them to last. All I got to do now are the boards. Just looking out there, there are a number of species. Just wanting people's opinion on what are the good species and whether the grades makes much differences (as I'm going to coat it with an oil with a tint anyway).

Deck size: 8m x 4m
Uncovered so exposed to sun 50% of the day.
House is an old Queenslander and I want to go with native timber rather than merbau.

Cost vs look vs durability and stability over time

Spotted Gum/Iron Bark - paying through the roof
Turpentine
Tallowood
Brushbox - apparently it twists over time?
Blackbutt
Northern box
Forest Red - not sure if this is a species or just random
Grey gum - same as grey iron bark?

Also, what is interlock grain and why does it matter?

Last question - will marine varnish work on decks?

Thanks for the advice in advance