I used some stop leak before - STP Stop Leak. It worked for a few months then leaked again.
I replaced fluid with new. It stopped leaking for few months then leaked again.
I used synthetic fluid before. It worked well (in my case) but steering becomes heavier than normal. Didn't like it so back to using normal fluid. And continued leaking.
I didn't go for rebuild because that depends on the quality of parts used, amount of rack wear & expertise of rebuilder.
My old rack (Odo: about 900,000 km) not only leaked fluid but have some play between rack & pinion gear. This because I neglected applying steering grease to rack. The steering fluid doesn't lubricate the rack/pinion.
I always check fluid level once per week.
So when I found a NOS rack I bought it immediately! (this after about 10 years of leaking)

As long as fluid level is maintained ok, you can continue using the car no problem. Sudden loss of power steering is usually due to leaking high pressure steering hose, NOT from leaking rack.

Amarin