Replace whole system due to refrigerant leak?
With some recent chilly weather we turned on our heat pump about 2 weeks ago and noticed that it was short cycling (turning on for ~30-40 seconds then turned off). We called a local HVAC company, they came out and found that the pre-filters were completely clogged. They were cleaned and returned, and all appeared normal. But that night it started short cycling again. HVAC company returned and now says our system has a refrigerant leak (determined by low pressure), either in the heat pump, furnace, or somewhere in between. He isn't sure where it's located or how slow or fast the leak is but he is betting that it's in the coils of either the heat pump (less likely) or the furnace (more likely) and said that he recommends replacing the whole system. He added some refrigerant to hopefully hold us for a while. The system has been off ever since (~3 days ago) because the outside temp has been warm.
We replaced the heat pump in 2007 (Bryant) and the furnace (Trane) is from the late 1990s (not sure). The HVAC company also said that we shouldn't have mismatched pieces, they should be the same brand (the company who installed it said it didn't matter, though they're now out of business).
He also said we can't replace just one part, it has to be all or nothing because apparently the type of refrigerant used in new systems is different from older systems and not compatible.
So...before we consider replacing the whole system, I just need to know is this information accurate, is there anything else that we can do to avoid replacing the whole system? Is it worthwhile to have him actually find the leak to determine if it's fixable?
Thanks for any help!