Changing from rigid to suspension fork
I don't believe you have much success in selecting a suspension fork. Looking at the bike, a suspension fork with exactly same geometry as yours would have approximately zero suspension. There is simply no room for a suspension fork with unchanged geometry. If you change to a suspension fork, it will change the geometry of your bike which will affect the steering characteristics. For example, the seat tube angle is changed (so you may run out of fore/aft adjustment in your seat post), the head tube angle is changed (so the fork trail affecting the steering characteristics will probably not be what the fork manufacturer intended), the bottom bracket height will change (so you will find it harder to mount and dismount due to increased bottom bracket height).
Every frame will have different requirements for a fork. If you see a fork, you need to carefully look at its dimensions and analyze what the fork trail will be and how the bottom bracket height and seat tube angle will change. Only with such an analysis can you determine if the new geometry of the frame/fork combination is acceptable.
A bike frame designed for suspension would have reduced head tube height from the lower end, making room for suspension below the head tube. For such a frame, the intended fork length is longer.