I almost exclusively just buy Dell OptiPlex machines from eBay and refurbish to whatever specific needs/budget these days.
Need a basic email/Facebook/etc machine? i3 (SB) with whatever it comes with, typically 4GB/500GB and DVD for movies.
Low-end gaming machine for kids or older games? i5 (SB) with 4-8GB, 160GB Intel 320 SSD (refurbished, $35), add in a 750 Ti (Dell OEM models don't require 6-pin plug) if needed.
Mid-range gaming machine? i5 (IB or even Haswell) with 8GB, same SSD, beefier PSU, real video card. For these, biggest challenge is removing the HDD cage from the case so can install a longer GPU.
All of these I've done recently have accepted the Windows 10 install without issues, even after the "free" period expired. Have set up probably half dozen in the last year for friends/family seriously on the cheap, like $100 for base i3, $150-200 for i5 low end gaming, around $300 for mid-range gaming.