"Last owner used hydraulic fluid" instead of oil. I suggest you call one of the places that specializes in parts for these machines and ask them what goes bad when someone uses hydraulic fluid instead of oil.
They have probably seen this enough that they may be able to narrow down what you need to look for.
Once someone has put hydraulic fluid in the system, what do you have to do to clean the system enough to make it reliable again? I don't know myself, but I'd be asking what hydraulic fluid does to the entire system and what has to be done. Short of taking the whole system apart and cleaning it piece by piece, how would you flush all of the hydraulic fluid out? Does it have to be 100% cleaned of all hydraulic fluid or can you flush it enough by repeated oil changes say run it to operating temperature, cycle the systems and then change the oil. Repeat. How many times?
These are some questions I'd be asking. Especially if the residual hydraulic fluid in the system can tear something else up if it hasn't adequately been dealt with.