Should I replace whole system or just leaky component?
If you check right away you may be able to find the leak with bubble solution, loosing most of a charge in 1 week is a very big leak and should be easy to spot.
I would not count on getting another fill of the R22 for 500$ that you got last time that was a really good price. If you can identify the leak it may be a simple fix so yes 150 may be worth it but remember you will have 500-1000 to top it up. If you can identify a leak with bubble solution I would get it fixed but if you can not find a leak time for a new system.