Wow, you are OC about the car (kinda like me)
- It sounds like the previous owner took good care of the car
- If I were you, I would simply set aside some money for repairs and not fix many things proactively at this point. Over the next 20k miles, I think your CV boots may go (unless both front axles are new)
- My answer to your questions below

Should I do transfer case and front and rear differential fluid change? Or just leave it till 90k miles at least? It sounds like some of this was done. I would change these fluids at 60-80K mile intervals
Should I change spark plugs? at 100k miles
Clean idle control valve? I wouldn't
When is water pump is due for change? depends, I changed mine at 60k (but it was leaking). When it breaks, it can/will starve the engine of coolant. The trick is to catch it before your engine overheats / warps. This might be the only repair I would pay for now if you can't find service history of it. Sometime dealers can use the car key to see if previous repair work was done at the dealers.
- I didn't see anything about the tranny oil being replaced. I would do this as well if you can't find history on it