Are freewheels and cassettes interchangeable? My bike is rather old and needs lots of work. I am trying to understand how to replace things myself slowly other than through a professional (to save bit money).

A bike company said that I need a new cassette now (I agree), so I bought a cassette removal tool and removed it now. 

On Amazon, seems a 7 speed freewheel is much cheaper than a 7 speed cassette. I am fairly confident that my current one is cassette, and just wondering if I have to buy a cassette or for the new one I can go for a freewheel? Will it be much more extra work for people without much experience? 

many thanks
No, freewheels are completely different from the cassette / free hub system and are not interchangeable. 

A freewheel includes the ratchet mechanism that allows the sprockets to rotate backwards as well as the sprockets themselves, and threads into the hub. 

In the cassette / free hub system the ratchet mechanism is built into the hub, and the sprockets slide onto a splined brarrel shaped ‘free hub body’.