I do have a K500 and while the K500s has a longer chassis under the sliding table to accept longer sliders, I am not sure it is precise enough for a DRO.
By the way, the outrigger and rip fence DRO is only available starting at the 700 level according to the Felder web site.
Between the K500 and kappa, you have 4 levels: 540, 700, 740, and 940. I am sure others would chime in, but I would look at 700 series if you are hesitating about a 500 series before going upper in the levels.
I would personally go for K700 if I need DRO.
I am doing the same stuff as you plan to do, but only as a hobbyist. I am fine with my K500 when I compare my results with what I saw made by professionals. However, if I have to do it professionally I would go with DRO for ease of use and speed. No need in my case, I can take time to get the dimension right by test and error :) and use a dial caliper to get my dado blade height correct.
Bill
