My used 5810 was originally configured with Win 7. Would there be an issue with activation if I go the MSFT download route? Thanks.
You might have to search that on the net... but I recall reading something about having a W7 COA is good enough to activate W10. Not 100% on that. (I bought my 5810s from a refurbisher who included an authorized version of W10 from their royalty license(?).) From time to time you can still see "how get W10 for free" on the net... might check into that if needed.
At one point there MS offered a free upgrade from Win 7 to Win 10. As @Scataphagos suggested, search around, there may still be ways to get a free upgrade since you have Win 7 license.