Focus RS block is exactly the same as the 2.0 litre block is exactly the same as the ST170 block but the roads and pistons are different for each given the different applications (turbo, NA road car, high comp warm hatch).

Do NOT rely on a poxy remap to be able to cope with a forced induction conversion on an NA car/ECU, it simply will not work (despite all the 'glossy-brochure-speak' to the contrary, it's bullshit!).

It all depends what you want, reliability or cost, except the two are connected to each other and you can't have one without the other. Supercharger conversions have been done (search out Big Will's thread in the resto/projects section for a proper supercharger build) with varying levels of success (again, determined by how much money has been thrown into them) so this route will yield a lot more information than a turbo conversion.

The biggest sticking point is whether to keep the VVT or not. VVT relies on high compression which kinda goes against what you need for a turbo conversion. The black oak ECU on the 170 WILL NOT be sufficient to run a forced induction conversion so you'll need something like Pectel/Omex/Aem which will need to be installed and mapped by someone who knows what they are doing (which is going to cost but it's fundamental to the reliability of the car!). If you decide to ditch the VVT then it's no different to any other Zetec turbo build thus defeating your objective of 'being different'.