I took my Nissan AD 2006 to Swat in 2016 and I think Sunny has the same engine QG13DE. Its a powerful engine provided its in good condition and gaskit is fine to withstand rigorous use. I mentioned gaskit as I had to get it replaced in Bahawalpur on my return journey.
My advice would be: Have it thoroughly checked by a good mechanic (telling him that it will go to a long tour) to check the suspention, belts, boots and hoses (as advised above), all lubes and specially its AC, besides regular things. In general Nissan is a strong car to use on rugged surfaces so no worry on its capabilities. Also, its widely used across Pakistan so in all likelyhood you would have technical support available.