Buying 10 ATVs for tour company in Thailand.. Advice sought

Hey everyone, thought this would be the definitive forum to ask for help on this. My wife and I are starting an ATV tour here in Thailand (we're American) and we'd like to know what you experts say are the best types of bikes to get. I pose these question to ya:
1) Air-cooled vs. water cooled (It's hot as hell here in Thailand, but these bikes will get used for about 6 hours per day, every day, up mountains)
2) Drum front/disc back , drum/drum or disc/disc (sounds like a no-brainer, but figured i'd ask)

Any other advice you'd have in my purchasing?
Please keep in mind these bikes will be getting used ALOT in hot climate, every day. I have about 300 offers from companies in Asia here with full specs and I don't know what is a winner and what is a loser.
Thx in advance
Kevin

go water-cooled and hydralic disc front and rear

If I was making the purchace. I would no doubt be Honda. They will last longer and have fewer problems than any other brand.

I would go with the disc brakes front and rear and water cooled, as for the brand I'd probably go with honda too for day to day use without many problems. Everyone has a different opinion on what brand is best.

Alot of my answers will depend on what size atv you are looking at.I would deff get some with disc brakes and if possible water cooled.Also an auto tranny would also be alot easier to drive for beginners as well.If you go with auto trannies then make sure and get a bike with a good EBS system on it so it will be alot easier to control on descents.As far as brands go,they are all about the same now days,you will get some good ones and some bad ones with all brands.Alot of people that I have seen with companys such as what you are doing use the Yamaha Grizzly.Very easy to drive,good brakes,good ebs system and water cooled.I have an 03 grizzly with almost 3,000 miles on it and it has never given any major problems.Only problems it has given was operator error.

my opinion: i drive an 07 yamaha big bear 4x4; i love the bike, great for any terrain and seems to be dependable; i would however recomend honda w/ straight axle for in-experienced riders, plus not a lot to work on w/select hondas(rancher), easy maintanence, light wieght, durable, a great bike; i ride a big bear cause a like the low 1st gear, irs, 4x4 locker, ground clearance; not a great bike for what you want i wouldn't think...just not the right bike, go with a honda, can't go wrong with a honda.

07 big bear exp edition