The difference with Lightjet is that you scan first, do all your necessary corrective tweaks in something like PS, then print. With an additive
enlarger, you do your contrast etc adjustments analog if necessary, generally by silver masking, then print directly. Different kinds of film have their own personalities. Portra 160 is not Ektar, for example, is not Velvia. But a correctly made colorhead should work for any of them. Obviously, direct positive papers like Cibachrome are no longer made, so directly enlarging a positive chrome is no longer a practical possibility. Understanding film dyes takes some experience, but in every case, your centerpoint for each respective exposing light must be the same. How much wiggle room from narrow band you need is a much more complicated subject, and you need to understand how dichroic filter behave relative to angle of incidence and operating temperature. The big problem at the moment is that if you invest a lot of effort into an LED system, it might be impossible to maintain, since bulb designs are going to rapidly change. It might be worthwhile to prototype some
electronics, but like I said, it's rather premature to think you'll find anything close to ideal bulbs yet for this kind of project. There are a number of people taking the project halfway, and designing two-color VC printing heads with LED's, which is a much simpler problem.