Gravity Heat Systems - Steer Clear?

[Sean Wilt]

Looking at my first flip and some of the properties I am looking at have "gravity heat" systems. Honestly, I had never heard of this!  But google says, probably ought to replace.    Should I just steer clear of these properties?   Not really sure how much is involved with replacement as some items have seen say there may be asbestos in the duct work.   These properties are older and generally on the low $ end so there is not much room based on ARVs to replace entire system but wondering if leaving it in would scare away buyers.