As you say you are ordering from vauxhall, what price are the c20let pistons?
When I was building my Zetec turbo (mapping out initial budget) I couldn't find any second hand vauxhall pistons and new ones were only £40-50 cheaper than forged Zetec items.

Also Big Horse power + BC gearbox = disaster.
I would look at IB5 or MTX conversions depending on how big your power goal is.