Um, you seem to be attacking this from the wrong end. Hate to be the bearer of bad tidings, but ---
First off, the formula for HP is for the output of the hydraulic pump and not the horsepower to drive it. Even then that is based on 100% efficiency and that ain't gonna happen in the real world.
Given that as a starter, there is too much to explain for this forum, well, for lazy old me anyway. You need to get a book or tutorial on hydraulic motors and start again. I would be VERY surprised if you 3.5HP engine is enough for what you are trying to do. No matter what the great deal on it was. I am guessing maybe 6HP or 10HP.
I have been wrong before, but you need to hit the books for about a week. Then come back and ask f you still have questions.