The ST170 manifold is NOT stainless steel - it's mild steel but it's a Cosworth designed item and is VERY good so you will gain nothing but pub bragging rights by changing it. It's the upgrade of choice for pretty much all 4 pot Zetec-E engines.

The Milltek sports cat/decat and flexi are meant to be good upgrades but there are questions about one of the mounting brackets on the flexi (I think) and, as you say, they are very pricey (Will Pedley was selling one recently, he might still have them for sale).

For reference the ONLY bit of an ST170 standard exhaust system that is made of stainless steel is the tailpipe tip.