Don't usually use hydrualic fluid for the suspension unless either the pump leaks or the accumulators are so bad the struts have been beated to death.
How hard does it ride in the rear? Hydraulic suspension should be very smooth, not bouncy at all. If it is bouncy, the accumulators are gone, need to be replaced.
Shouldn't be a problem at the age of your car.
yes, the hydraulic fluid is expensive. No, don't use anything else, you may never have to add the whole bottle.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!