The BX will do everything you asked for, you just need to add liquid ballast to tires for your slopes. The tracked equipment you are looking at moves slower then BX. The BX goes up to a little over 8 miles an hr and ASV PT30 goes 6. Keep in mind, when adding implements like rear rototiller will make the whole thing much more stable. You mentioned areas already terranced, no need for track. However you say you have large boulders, the BX can move a good sized boulder, not boulders.
You need rent a heavy duty exacavator with a hydraulic thumb and move what you need to do for the day. I think its safer and cheaper to do it this way so no time is lost from damaged equipment and being hurt from hills.
As for your garden, its nice to use the FEL or trailer to carry stuff from garden from the terranced area. I am not sure if I want tracks all over my yard towing a trailer or carrying gardening tools.