Save your money. If the wheel is otherwise OK (aligned, no spoke breakage issues) an axle replacement is a quick and inexpensive job (one that you can learn to do yourself), and you'll be as well off as with a freehub and cassette, which will cost much more.

Since you've confirmed that the chirping is only when coasting, then either the freewheel needs oiling (another easy job) or a part of the broken spoke protector is rubbing the backside of the freewheel down near the hub.

You should find a loal shop that can replace the axle, and do the rest of the related work for a reasonably low cost. I don't know where in the UK you are, but if you're in a larger city, there are probably bike co-ops where you can do this yourself under supervision and guidance (learn as you go) or have it done for a low price or sometimes in exchange for volunteer time.