There are several issues here.
The main one is value for money and what do you want to do.
I wouldn't get an instructor's rating if you just want to build hours. You won't do your students any favours.
How much does an instructor rating cost?
How much will your next IR renewal cost?
Add them together and come up with a number. (Add at least 20% for stuff ups and delays)
Can you live on instructor pay?
Now, how much will a Shed rating cost? Good question. BAC Express and Emerald do them. I think you are looking at about £8-12,000. (Depending).
While both companies are in a bit of flux at the moment with the loss of the mail contracts they have still been recruiting. I don't know when Emerald are going to run another TRTO and I am not sure what BAC Express are up to either.
Will you get a job out of it? Probably unless you are a complete numpty. The only "if" is when. Both companies are losing Captains and FO's to bigger turboprop/jet operators.
Advantages.
Turbine aircraft, JAR 25.
Multi crew experience.
IR renewed (multi crew only though)
More night flying that you can poke a stick at (once employed)
Every one moves on to bigger toys and freight always needs to fly regardless of what's going on.
Disadvantages.
Training can take a while as charter/sheduled flying comes first.
May have to wait a while for a start.
Type not that commonly used any more.
Pay ain't brilliant. (~ £14K plus allowances for the first 6 months)
I forgot to mention, if you want to work in Australia then you'll have to convert your licence to an Australian one. Not that big a deal compared to coming the other way.
You'd also have to check out the Immigration details. I think there may be a working visa you can get depending on your age.
Be warned, there are a lot of low houred pilots chasing jobs there too. Best you can hope for in oz is either instruction or bush/scenic flying etc. No jet/turboprop jobs for low houred guys. (with the exception of those going through cadet schemes).