As I said earlier, you can't start at $1 so where else do you start?
I agree that it will be difficult to reach the target price but I also think you'd be doing yourself a disservice to list it at 20-25k. Why not put it out there as the seller has and see what happens? It doesn't need to be perfect to gauge market interest and if it doesn't sell you can try again.
I'm one of the first to bag silly auctions but I don't think this one is silly. Unrealistic? Perhaps. But it's not insulting anyone and I've communicated with the seller and he seems 100% up-front and genuine. I'm pretty confident he'd negotiate with anybody that was genuinely interested.
Provenance is an entirely different topic. All I know about the build is what I read here and it seems epic and much, much, much more effort than you can see on face value. But it's ultimately a chop and I have no idea whether it's all new bolts and bushes, all torqued to factory or not so can't comment on that (other than to say the only way to do it properly is to do it properly)**.
If somebody asked me (and they haven't) I'd hazard $30k, or perhaps $35k to somebody in an aligned trade that could use it for marketing. $22k for the M3 bits and $8k for the uniqueness. Assuming subframe, VANOS and rod bearings have all been done. I could be miles off also but you'll never recover the labour involved (and if HellBM did then they should be laughing all the way to the bank).
I seriously hope the right people meet up to do the right deal on this one, it's pretty damn cool. I want it badly.
** Somebody I know purchased a chopped E36 hybrid thing from somebody who should know better and found it had basically been put together with whatever was lying around. Standard bolts where HT bolts should be, used bushings, missing washers, poor torque values etc.