Need a new exhaust

[JumboBeef]

My 91 RRC V8 auto LPG has just failed it's MOT on the exhaust. It is blowing on the down pipies but to be fair the whole thing needs replacing. The vehicle is completely standard and is a 'keeper'.

Do I:

replace everything but the manifolds in mild steel?
replace everything but the manifolds in stainless steel?

Should I replace the manifolds at the same time (someone once said to me that was the the quickest way to increase performance on a standard V8).

I would like a bit more V8 noise but I do not want anything boy racer.

Any suggestions please!

Thanks.