8mm is the new 8.5mm i.e. 8mm means light but not durable but 8.3-8.5mm means durable, but still pretty light.Dry treated doesn't just mean dry treated. If you're going neat water (liquid or solid) it's a must, but even if not, it will mean the rope lasts longer - it reduces the amount of grit, and dirt which can get into the core.As for length, I'd go for 60m. Half the time you will hate it, but the other half the time you (or your brother) will thank God, Allah, and me that he did. He may even find it saves his life (multiple abs reduced, especially if bolted for single 60m rope).For most Gritstone outcrops, a single 60m rope doubled over is sufficient.
always better to have too much length than not enough