Gearbox oil change / steel pan or not?

Have found Motospecs (do net search for local branch) to be easiest and reasonable price one stop shop for Lifeguard 6 and steel pan/filter kits.
Imparts automotive (do web search) also have BMW steel pan (same) at good price. 20L drum seems best price option for fluid buy.
What you don't use of a 20L drum of fluid (after bit of a flush) will be used next service, but keep lid on tight.
And Toyota gasket sealant is voted best option to prevent leaks on sump (not cheap, but it works).

For what it's worth, Nulon Australia claim their fully-synthetic ATF is compatible with the ZF 6HP26 and they reference the ZF equivalent spec (Lifeguard 6).

Refer - ZF Lifeguard Fluid 6 *# - Nulon Products Australia

A mate of mine flushed and refilled his FG Falcon (same trans) with this oil and has put quite a few miles on it now without any issues. But that's hardly an endorsement, just that the trans didn't go strange after getting this oil.