The way to figure out overlay is to measure the width of your cabinet opening then measure the width of your door. It's easiest if you do this on a cabinet with a single door. The next step is to take the door width minus the opening width. Then take that answer and divide by 2. That will be your overlay. Example: door width = 13" and the cabinet opening is 12". 13-12=1 then divide 1 by 2 and you get 1/2" Overlay. If you need more help, let me know.
If you are replacing existing Blum hinges, you can get by with replacing one hinge with doors that have 2 hinges. If the door has 3 hinges you need to put 2 soft close hinges on the door. If your existing hinges are not Blum hinges you will need to replace all hinges. The geometry is different on between brands.