For your experience and weight I'd say something around 26" would be fine, it may take a little adjusting to get used to and a 28" wide board will be immediately more comfortable but going narrower will be faster and more rewarding in the long run.
I was keen on doing a bunch of flat-water training for fitness and to drop a bit of weight and took a punt on a 14'0"x 25" pure flat-water board, the first time I paddled it I thought "what have I done" and it did take me a while to get used to it but now I feel very comfortable even in crap conditions and feel I have become a much better all round paddler because of it.
Carbon V Other Constructions (As there are quite a few): To me it really depends on the manufacturer and whether they are looking to make the lightest possible race board purely for racing/team members to get on the podium (and sell more boards because of it) or a lighter construction race board that also offers durability for the general public. I had a very well known brand 14'0" board in full carbon that was great to paddle, felt great under the feet was easy to transport etc. but you only had to look at it and it cracked, I've still got it and love paddling it, so probably won't get rid of it, but it has been sitting on a shelf waiting for repair for the past summer and unless they have changed their manufacturing I would never buy another one again. I've now got three carbon boards from the same manufacturer and wouldn't go back, they aren't as light as my previous carbon boards but lighter than epoxy and IMHO just as strong.
As for your knee: I haven't ever done and acl (and hope I never do) but I did have all of my cartilage removed from the right compartment of my right knee in May last year and it took me a while to get back to my best, but all I can recommend is find a good physio, do the work stick to the rehab program religiously build up the muscles around the knee and you should be fine. If stability in the knee is in doubt go for a wider board and sell it later for a narrower one when you have got all the strength back.
Check out this vid RE: Race boards and weights.