I was £300 from the cat-back at a custom stainless exhaust builder. Asked for a deep tone and not a boy racer loud sound. Sounds awesome and does have the burble. Not a turbo burble clearly but a distinctive boxer burble.
You can get a pair of headers on ebay for 250 but it's thinner stainless and with stainless being a harder metal is more prone to cracking than steel. I have however had stainless headers on a previous car and had no issues.
I'm not really sure how much point there is getting stainless headers though, as the gx is tuned to about as max as it'll go. Full stainless, filter and a remap will be lucky to give you about 5BHP. That verses the cost... sell your gx, get what your willing to spend on it and get a turbo.
I'm happy with my gx, has sti alloys and a nice exhaust but I too need to be realistic on what to spend on it.