http://www.ebay.com/itm/MICOMP-Dell-Tower-Desktop-Computer-PC-Dual-Core-3-4Ghz-4GB-250GB-Windows-7-64-/271876179198?hash=item3f4d1544fe:g:x6YAAOSwI-BWJmY5
Small-ish PC, running windows 7.
Just set the BIOS to ignore errors, such as no keyboard/mouse/monitor.
Set BIOS to turn back on after power failure.
You can also set the auto turn on time for daily in case it is powered off.
Disable windows power saving functions, or use a service that supports wake on lan.
Then put on a remote desktop software, such as chrome remte desktop, with the clients permission, of course.
You could also get a $100 HP stream, which runs windows 8 > upgrade to 10, but the CPU is so low powered, than many plugins probably won't run. But it is tiny...
http://www.amazon.com/HP-Stream-Windows-Certified-Refurbished/dp/B015WSH4VC/ref=sr_1_4?s=pc&ie=UTF8&qid=1445533900&sr=1-4&keywords=hp+stream+tablet