I know water tanks are supposed to be glass lined but I have to wonder, whats more nasty, glass in you hydraulics or glass in your cup of tea? I would go with the water tank, weld on a NPT bung large enough to use a suction filter, this should prevent the glass from getting into the system if there really is glass rattling around in there. You will probably have lots of rust to clean out. You could have four holes in your tank for water, electric heater and pressure relief. You will need an outlet positioned near the bottom and an inlet about 2/3rds up. Without a baffle, I would mount the inlet perpendicular and as far away from the outlet as possible. Also you could consider an old air tank? I used this one to make a gas tank yesterday for my tracked dumper project.
If I weld on this glass-lined tank, wouldn't the glass shatter?. As fas as I know there are no loose glass particles in the tank. The application is for a tractor. I am not firm on this idea, but it is an option.
I like to find a tank to fit behind the seat, maybe 30-36" long with adequate sides to make a 12-13 gal. capacity. I can weld some but I am not comfortable with 12 gauge material and a leak proof project. I guess I can always go to a welding shop.