.... one more question - I see Lefty forks listed with different travel options 100mm, 110mm, 120mm and 140mm (I refer to 26 wheels as I believe the same forks can be use for 29 but with reduced travel so the tyre does not bottom out).
Are the fork externals the same for the different travels, the travel being set internaly or do they have longer sliders (if that is the correct term).
Or more specificaly, if I get a 140mm fork, will it weigh same as 100 mm equivalent?
No the 100 mm and it might even be 110mm will be a little lighter than the 140 mm as the housing and internals are slightly shorter. Roughly 120 Grams on the carbon fork and close to 200 on the aluminum.
Yes the same fork can be used on both 26" and 29" frames but if the fork has no spacers then it is set up for a 26" frame and if it is used for a 29" frame you need to put spacers in the top that limit the travel (normal 30mm) other wise the fork can compress to the point where the tire has contact with the bottom of the head tube. Then you have one serious endo. Also when you put spacers in the top you need to put volumizers in the bottom. They make the air chamber smaller so that the pressure curve of the fork when compressing is even for best performance. An example of what Canondale does if you look at two identical forks one with 140mm and the other with 120mm or 130mm they will be the exact same fork and the only difference is the travel spacers and the volumizers. you can also take a 120mm fork and convert it to a 140 by removing those two parts. The top travel spacers are very easy and accessible right under the top cap the volumizers are at the bottom and require some specific tools so it is best to have the job done by a person who knows lefty forks.
For a 26" bike unless it is a true downhill rig there is no reason for a larger housing in fact it will be a little to long for your frame affecting the steering angle.