AI wasn't changed from Native at all, I'm afraid, and as far as I know it is something that has been largely ignored by the modding community (I may be wrong).
Warband AI (while sieging) is a huge step up from M&B original - in M&B the Lords break off their siege and chase if even a couple of farmers pass them by - so from that point of view I couldn't recommed that you buy the original M&B.
However, if you can get past the AI, obviously there are a lot of mods that are pretty well rounded in other aspects, and if you can pick up M&B cheap, why not get it?