It's cheaper if it's a freewheel. The shimano freehub body price itself exceeds the cost of most 5/6/7 speed shimano freewheels. And since it's one unit, it's just one replacement cost not two (freehub + cassette).

Depending on how worn the chainring is, that might give an indication to how worn the chain is. Visual inspection of the chainring/chains + measurement and/or test ride for skipping will give you a really good idea on drivetrain condition.