why not install something like teamviewer on the old computer and then you use just the main PC, but can remote onto the other PC when you want to? - no need for KVM switches? only time you need to have a keyboard in the old machine is when you boot it up. You could also then use both monitors on your new PC (and have your Teamviewer open on another)
Remote desktop (MSTSC) is also built into Windows and would do the same thing. This is a better solution, as you can copy files easily between the 2 machines (although Teamviewer allow you to copy files i find it very slow) and you do not need to have internet access in order to log on to Teamviewer. your machines just need to be connected to the same network at home.
I use this keyboard Logitech K780 Multi-Device Wireless Keyboard with Silent Typing Create and communicate on your computer, your phone, or your tablet with the Logitech K780 multi-device wireless keyboard. Connect up to three devices. www.logitech.com And this mouse Logitech M720 Triathlon Multi-Computer Wireless Mouse Seamlessly create and communicate across up to three computers with the M720 Triathlon wireless mouse featuring programmable buttons and 24-month battery life. www.logitech.com
Both are multi device, works with windows , unix, osx and ios and over both wifi and bluetooth ....beats a kvm switch by miles