I think a snorkel is pretty important for a Cape Trip but there are ways around not having one.
With regards to the lift kit, replacing shocks and springs is usually done to cater for a better ride when fully loaded and to cope with corrogation. Due to the stronger and larger shocks and springs will give you extra lift. For your BT50 we are prolly only talking about 2 inches max. Coil spacers can also be used to give a little extra lift. I think that once you replace your suspension setup you will have difficulty selling it second hand. Not many people buy suspension 2nd hand and if they did they would want it very very cheap. With the labour involved it may be better just leaving the modded suspension on the ute.
At the end of the day, if you load the Bt50 safely, drive it carefully, travel in the dry season, travel with other 4x4s and recovey gear you should be able to do this trip without a snorkel and suspension upgrade.