A clean install of Windows 10 will activate on the MB if installed, as the COA is embedded in the UEFI.😀
Chances are, it shipped with Windows 10 Home, although could very well had been shipped with Windows 7 Pro with the upgrade option to 10 when desired, the DVD was included in some retail boxed units. I'd try 10 Pro first, or 7 Pro if necessary for upgrade.
The real issue being if the 8900 MB it's an exact fit for the case, if needed, there's extenders for various cables. Plus the M.2 NVMe connection on the 8900 is 2nd gen (10Gb/sec), that can be fixed by using a $15-20 PCIe x4 adapter for the NVMe SSD & run in the 2nd GPU slot, as it's x4 anyway. The XPS 8700 has a 'fake' x4 slot, had I known that, would never had purchased the PC. The original specs were corrected & the old completely taken offline as though was never published.
At any rate, the MB will activate Windows 10 just fine, or possibly Windows 7 Pro, as it was sold on these at Costco. If it did ship with 7 Pro, will upgrade to 10 Pro. Meaning one can clean install 10 Pro w/out issue. No COA has to be purchased for OEM MB's, the reinstall media are plentiful on eBay & maybe even Dell will ship the media for a cost (just say you've lost the media). Dell has sent me at no charge & with overnight shipping reinstall media for XP Pro computers well over 5 years old three times. Of course, all were the same anyway, I didn't open the last two when I noticed the P/N was the same.
Once booted into the UEFI (or BIOS), the Service Tag of the original PC will be shown, as the case with all Dell computers ever since my first one in 2003. That Service Tag will then take the place of the one on the XPS 8700 case & one can lookup the exact configuration, what OS & version (Home or Pro) it shipped with, extra software, drivers needed, etc.
I have an XPS 8700 myself and am tempted to go for this upgrade. If I don't, the sole reason will be that as always, Dell cuts corners. It would had been great of them to spend the extra $2 to have twin 4 pin CPU power connections for more power delivery, as sites like Intel HWBOT will cause the CPU to throttle bad when benching. I discovered this when upgrading my i7-4770 to a i7-4790K, as Dell released a UEFI update for unlocked Haswell CPU's. So I removed it from the Dell and built a Z97 machine with the ASRock Z97 Extreme6, now my main PC.😀
Plus it couldn't had cost that much to have the Ultra M.2 (32Gb/sec) slot, rather than previous gen. ASRock had introduced it long before the XPS 8900 was released, as did other OEM's. Dell could has spent the extra $2 to make things right, customers wouldn't mind paying an extra $10 per PC for the full experience.
Being that DDR4 RAM is always on promo now on Newegg, if one can find a bargain on a Skylake CPU, preferably an i5/i7, it's a worthwhile upgrade if the cost can be held around $350 total. There's some good deals to be found on this MB & a CPU on eBay. As low cost as RAM is, with a lifetime warranty, I'd say buy new, unless one finds a seller of the MB & RAM is included.