The question is will I have the option of doing a brand new clean install of Win 10?
Since it's free I would assume You could do a fresh or upgrade install. In my case I just upgraded from Win8.1.
If so does that mean I can then clone the OS partition and then copy that to a new computer that I will build? I know I would probably have to call MS to get it authorized but will that work and thereby save me from buying 8?
Again, assuming the free Win10, no need to clone the OS or call M$. Just copy the data you need and build the OS fresh.
Or, am I being too clever and I should just build the new comp now, buy Win 8 and then do the free upgrade later?
Depends on how long you want to wait for Win10. I haven't looked at any of the release date estimates lately but I would assume no earlier than fall. If you want to save $$ wait for the release.
What SSD/mobo do you guys think is the best bang for the buck?
For the SSD, I'm not particular on brand names. I just look for what's popular and had good reviews/prices from Microcenter and/or New Egg. When I bought my 250 Gig SSD for Win8.1 OS, the 1+ terabyte drives were coming out allowing the 250s to drop in price. Go a couple of stages below the biggest size to save more $$. Mobos, I usually don't worry on that, I usually purchase Dell refurb PCs and keep them for years. My current Dell Precision Engineering Workstation (4 core) was new in 2008 and I've only had to upgrade memory and hard disk. It runs fine for any audio stuff I do.