Identilynx has at best 60 fps so time resolution for it is not great. It is primarily designed as an identification device, not a timing device. It will work without a Finishlynx camera.
Without a finishlynx camera you cannot introduce a start signal, photocells only connect to Finishlynx cameras. Identilynx cameras do not have internal quartz timing crystal so the time shown is from the clock on your computer, not too accurate.
I would probably go with the photocell start and would look at a low pressure tape switch to put on the top ladder rung to get a finish. Overall a much less expensive solution and easier to manage. It also gives you instant results without having to analyze. You could also modify the ladder with a hole just above the top rung and use a photocell there but a tape switch would be easier.
Clock, Photocell and Tape Switch for about the price of a 60 fps Identilynx. Buy a clock and photocell that has expired homologation for cheap and spend a few bucks on a display board or two, much better!