Nasher
If you want to go the Teach Your Self route, a TIG inverter is available from Halliday Tool Loan (Un)Ltd.
I have Argon, MS and St/St rods in stock. You will need to get some new tungstens, cos mine is are little short and you will probably go through more of them than filler rod to start with.
Sorry its not an AC machine, so no ally.
As JW says, if you can do gas you can do TIG - with a little practice.
When you get the hang of it you can teach me.