Your mechanic will make money out of your pockets!!!!
Change the brakes
if finished only. New brakes
won't improve your towing capacity. Same for the clutch. Shock should also be replaced only if needed. Shocks in good condition won't also improve your towing capacity but only allow you to tow according to the truck limits. That's it that's all,
As an educated guess I would say that a loaded 16-17 FGRV would weight
near 2700-3000 lbs.
Your trailer must also have brakes
That normally requires a 6 cylinder motor AND an over 3,500 lbs capacity. I would be personally comfortable with a 4.000-5000 lbs towing capacity.
You Nissan would be ok ( I would prefer a six) for a 13 feet trailer.