I would recommend you check out the site "Stuck in Customs" (stuckincustoms.com). It is a site run by professional photographer Trey Ratcliff. On the site go to the tab "Stuff you need". He has quite a few good recommendations - and some fabulous HDR photos that you might enjoy.
I use the Manfrotto 190CXPRO4 when traveling by air as it just fits into my suitcase. However, my heaviest lens is only about half the weight of the Sigma 150-500. Since you will be mountaineering, wind is a considerable factor to consider. Although it is easy to carry and folds up nicely, I think you might want something a little heavier and stronger than that particular Manfrotto. Right now I think some of the best bang for the buck in carbon fiber is with some of the Chinese brands (there are several that seem pretty good). With those you should be able to find something that is closer to the strength of a Gitzo, but closer to the price of a Manfrotto.