We didn't bother getting one fitted and I never felt I needed one. They might be necessary at the beginning of the dry season but, in my experience, not at the end of the season. As usual, it depends where you want to go. I can’t comment on what is required on side roads off the OTL or bypass roads e.g. Lockhart River or the Iron Range National Park. It appears to me that a lot of snorkels fitted to vehicles in Australia are more of a macho accessory than a real necessity.
If you were to take the bypass roads, it would be possible (but probably not advisable) to get to Cape York in a 2WD vehicle at the end of the dry season. It would take a sturdy vehicle since the corrugations in places are some of the worst I've seen anywhere in the world. The worst at the moment are on the Southern Bypass road heading north between the Heathlands Ranger Station turnoff and when you rejoin the route of the Old Telegraph Line - about 55 kilometres of the corrugations from hell.
Better to have a 4WD and then you can bypass the bypass ! Although everyone seems to think you need an expensive 4WD with a gas-guzzling engine (read Landcruiser or Patrol), this is definitely not the case. We did it in a Subaru Forester X in a minimalist style. No trailer, no roof-rack. If it didn't fit under the cargo blind at the back, it didn't go.
Reading the books on Cape York and following their extensive lists of ‘vital’ spares and tools, you would spend more on spares than the whole trip would cost. I’m not sure whether this is because they really believe it or because they wish to avoid the attentions of those boils on the backside of humanity, the legal profession. Most seem to recommend spare hoses, drive belts, filters, coolants, brake lines & fittings etc etc. Our vehicle is only 6 months old and when I went to the dealer in Cairns to seek their advice, they advised me to take nothing ! Good advice. We took nothing. Sod’s Law says that whatever you carry won’t be the thing that goes wrong anyway. They said that there were regular delivery trucks heading up to Cape York and they could get anything I might need to me in about 2 days. Good enough for me. Anyway, I would have no idea how to change a drive belt even if I had the spare and tools. My advice is to forget them and join the RACQ instead.
Luggage and bags: Before you leave, collect everything you think you need and spread it out on the floor. Then put half of it back where it came from. You should now have everything you really need and you won't carry a lot of unnecessary junk.