I've been running this game for about a week now and while I haven't finished one single segment (only died once last run - getting there) yet the times I'm getting are sub 1:10 so I'm looking at recording options. Seems pretty clear that I'm going to need a Gameboy Player on a GameCube however the disks for GBP disks are outrageously expensive. Looking for alternatives I came across this thread http://www.gc-forever.com/forums/viewtopic.php?f=37&t=2782 which should allow me to boot into the GBP much much cheaper than the standard disk.

Before I go aheas and do that I want to confirm, will times submitted using this method be considered legal?

Yep. Plenty of runners (in both Fusion and Zero Mission) use GBI.

Nice one. Cheers.