Used and running fine with documentation NASCAR engines are fairly common in North Carolina. Expect to pay $12k to $40k for one, which is quite a bargain actually. I imagine in the UK this would translate into like 40k Pounds or so. I need to start and export business to the UK....
Someone happened a few years ago and there was a flood of heads and complete engines on the market. I think it was a change in head design tha caused it but I don't keep up with that stuff.
If interested search for used NASCAR parts in the Mooresville/Charlotte area of NC. A few outfits deal in that exclusively. The Lola's fueling system and dump cans came from just such a place.
Once this project hits the track there 'should' be a small flood of windsor based 358 motors on the market, but they might phase them out, ie keep them for a couple of seasons in trucks & ARCA.