After building quite a few of these converted cars for myself and customers over the last 15 years. I can honestly say that not a single one has sold for less than what it costs to build. Lets face it jay your talking about some random mates focus that you say sold cheap. But who wants a mk1 focus at the best of times let alone cosworth converted.

If it was a nicer car to start with like xr3 or s1 etc then it might of be more worthy.

As for comments on sierra floor pans or even the rear beam... depending on how much you want to spend you can sort anything out. Wrc rear beam setup is not that much anymore now there are so many copys available.
Floor pan you can make your own. Not that I see any issue with the sierra one. If your that bothered then 6 link the rear using mk1/2 setup.

There will always be someone who want a cosworth converted car as long as you choose the right shell to start from.