Originally Posted by IPO
Thanks for the fast repley nizwiz! ...
My 2 cents on your questions:
1: You'd certainly be better off starting with a newer game, like HL2. Q3 based games are pretty much history. The only part of JA that's still noteworthy is the swordplay.
So, if you're not interested in the swordplay aspects, I wouldn't recommend bothering with the JA source code at all.
If you are interested in swordplay, you should really consider going with OJP enhanced instead, since it's swordplay is - well - enhanced, and quite so. And OJP is going to stay a considerable while longer, I suspect (v 1.2 has recently been released, and they're now working on 1.3; it's getting quite some attention at the moment, I guess).
2: First: With "vanilla JK3" I mean just the source code as released by Raven in their SDK, as you can download from the posted link (you know, the basic stuff, without any extra flavours - like in "vanilla ice cream"). OJP is itself a mod that has started off this source code and changed it in two levels ("basic" and "enhanced"). Like XycaleTh said, the "basic" version is mostly bugfixes, and the "enhanced" version is "basic" plus a lot of quite hefty enhancements (so hefty that it'll be a whole new experience, at least when it comes to swordplay). As opposed to most code mods out there, OJP's source code is available to anyone to use as a basis for their own mods, more or less (I don't know the exact terms of their "licence", though).
Now to your question: Probably, no mod you create via coding will work with any of the mods you mentioned (regardless off which code base you start), since those mods have their own code bases and there's no (easy) way to combine code mods other than merging the source code (the only scenario in which two such mods could work together would be if one were a server-side only mod and the other a client-side only mod). Still, if you think you're gonna end up merging your source code changes with other mods (which is unlikely, since it would require you to get the source code of those, and that's no easy feat), vanilla jk3 is the safe bet (otherwise your code change might depend on some feature of OJP that the other mod lacks).
3: Like XycaleTh said. You won't need any of the professional features that the express editions lack, to be sure.
4: I cannot say (haven't tried to compile OJP myself, yet).