Bob,
In your particular application "freewheel" is not needed. I suspect that you want the empty trailer to return to the bottom under control. You will have to power it in both directions and I'm told hydraulic motors are great for that.
You could very easily find out the gear ratio of your winch. Just start turning the dual sprocket and count how many it takes to make the drum turn one full revolution. That number will help with figuring out motor size and eventual ft/min cable travel speed.
The one thing I would warn about is that these winches (even the new ones today) have a maximum recommended input RPM. My winch was made in the late 60's and I could not find out that number, but I assumed I want to stay well below 1000RPM. You can buy hydraulic motors that are well below that RPM range.
As far as power. To pull that 400lbs trailer you will only need a small fraction of the true pulling capacity of that winch. You can buy a huge variety and HP output of a hydraulic motors and can always make the final adjustment to winch speed by selecting the proper sprocket ratio between motor and winch.
I'm sure you can power it with the
B7500. I have zero knowledge about hydraulic motors but there are people on here who know a great deal. Try to post about it in hydraulics.
