Now, on to your questions....

For water, I also use a hydration pack. I find it just easier to deal with. I find reaching down on tight single track difficult and it's easier to put my spare tube, tools, etc in. When I first used one, it caused a back ache and was unconfortable, but after adjusting it some, I forget it's there. Also, the bottle does get dirty, but if you don't put your mouth on it and just squeeze the water out, it isn't bad.

For night riding, to keep it short and simple, I would recommend a 10W halogen bulb. They aren't too expensive and provide adequate lighting. 5W halogen won't really be enough light, a HID is awesome, but expensive. I currently use a NiteRider Evolution and it works well, I have never felt I needed more light. My husband is using the CatEye DoubleShot and is very happy with it's performance.

I cringe when people talk about hosing down their bike. I honestly never do. I let the mud dry on then just wipe it off with a rag. I'll use bike cleaner and rag to clean the frame and then lube up the drive train. If you must hose down your bike, just don't use pressure and I would recommend lubing up the cables afterward. We have customers that neglect their cables after hosing off the bike after every ride and their bikes look clean but the cables are all rusted in a few months. Just drip a little lube on the cables near the housing and shift or squeeze the brakes to work it in.

I only carry one tube. I rarely get a flat. Actually, I get more flats on the road than the trail.

Play around with saddle height and see what you like. I keep both road and off road bike at the same height.