Huh.? I had submitted a reply... Well, I'll send it again.
Basically the idea is to adapt as best as possible D&D to an MMO. Since I love L2 and have played it for several years, I decided to try and adapt it to L2. I don't have the time and team to start a new game from scratch. I think this mod will bring new "life" to L2, which, to me, has become a bit tedious due to the lack of builds and versatility of the characters. Core PnP (Pen'n'Paper) D&D players might not like it because it will have to suffer several modifications to become playable in L2, but if you look at DDO (which, may I add, is an excellent game) you'll see that they had to alter MANY things to make it a fun MMO. So finally answering your first question: yes, the idea is to capture the essence of D&D, not make it a "Pen & Paper"-like MMO (if that is ever possible some day).
About client changes... Look: officially only server changes, but I think we all know that a modification in such large scale will necessarily need client modding, but I can't really talk about that on the site. That's only 'backstage' information...