I'd say go for the 2litre model as you can quite easily add the best bits from the ST170 from a tracktoy point of view.
such as the brakes and airbox/air/feed.
with a decent exhuast, springs/dampers and maybe a nice remap the 2litre car will be at least on pace with a ST170.
As mentioned above as well most other parts will be cheaper/easier to get hold of and the 2litre standard engine has a lot of tuning options should you want to go further be it turbo or or n/a tuning.