Building any rifle is difficult, when you consider carving and simple engraving that makes a longrifle special, I don't think that a Hawken is any more difficult to do than any other quality build. The key is in the details, the form of the Hawken is extremely important. Unless you wish to make a bench copy of a specific rifle you have some latitude in your build. I believe that small seemingly insignificant errors in the lines or shapes of the area of the tang and the gracefulness of the shape of the forestock are hard to explain but very easy to see on a completed rifle, if it's right it feels light and graceful, if not, it is a fence post. You won't know unless you actually build one, but if you purchase a "kit" I would recommend a stock that does NOT have the lock inletted as the placement of the snail/lock is critical to a good start on the rifle and precarves are not always cut correctly and sometimes require a great deal of work to correct. I enjoy Hawkens and have built three this last year, one with a Bill Large barrel, one Colerain, and I just finished a GR Douglas. My next project will be an original GRRW Hawken kit with a .58 tapered slow twist barrel that was in a closet in the original owners house for 34 years, untouched since it left the factory.
My advise is jump in, do the best you can, take your time and learn from experience, I am sure that if you do, you will enjoy the journey.