Do you think its worth purchasing Museum , batobus and open tour bus tours passes ahead of time? Or is it easy to purchase once you are there?

No, yes

all are cheaper when bought there. Museum passes at any museum - chose a lesser one for no lines. bus and boat buy upon entering.

so very easy to purchase there and no reason to buy ahead. If it's a Sunday quite a few museums have discounted rates and some are free on the first Sun of the month (Louvre)

No, Yes as Alan said. Having said that though , I often pre-order various museum and exhibit tickets thru FNAC and pick them up all at once in Paris.It often saves standing in line for popular exhibits.

No
Yes

Do remember that the Museum Pass does not cover special exhibits!