I have been looking into this also for my Mk6 although I have the TD which I love in normal driving but on the dual carriageway its like "get it out of here!!!!" The ST170 is a Zetec derived engine I believe and from what I am hearing if you can use the donor car loom and other parts from a standard Mk6 (think flywheel and sump) then it will be a less complicated modification.
Of course I am unsure of the engine and gearbox mounts will line up so you may need to get new mounting plates and have an old Mk6 mount adapted to fit the new plates/mounts.
As for the ECU several I have seen use throttle bodies but if you have the full loom from the ST170 why not use that. I think you may find it difficult to get the fuel tank for the van now as the petrol pump is different to the diesel and there are not many of the petrol variants around nowadays. You could modify the diesel fuel tank tho with much caution!!!!

I came across this service on eBay for original car looms to be modified to run the ST170 if you are unsure how to do this yourself.