Prep is the most important thing. Are you painting over raw wood or has it been painted previously? Not knowing what is already there, or the condition of it, I would suggests sanding it down to either bare wood, or to a smooth finish of whatever was used to prime and paint it originally. Old paint can be the best primer ever if it's already proven that it's bonded with the wood and lasted for decades.
Since this is something that is going to be intricate, time consuming and you want it to last, oil based primer is the very best. I prefer Zinsser BIN. It goes on like water, which makes it very messy. It sprays real good if you have a sprayer. Sherwin Williams also has a very good oil based primer for exterior use that is worth the money. There is NO OTHER brand that I would use.
Latex paint sticks to oil base primer really good. Depending on the number of colors you plan on using, oil based paint is always going to be better then latex. I've never tried to paint a pattern with it, so that needs to be considered. If you go with latex paint, stick with either Valspar or Sherwin Williams. Those are the only two brands that survived the reformulation a few years ago. Whatever was good before then, doesn't mean anything today.
Clear coats can cause more damage then the prevent. I doubt that I would put one over the paint. In fact, it would take a lot of convincing to get me to do that. I would talk to the people at Sherwin Williams before doing it, and even then, probably not do it.
