I think you'd better not to buy Kyosho RTR car (both 1/8 and 1/10) if you may plan to change engine in the future, becaue you will spare $300-$400 to buy engine mount, clutch, flywheel etc..... I would recommend you to buy a Taiwanese buggy RTR if you really want to play off-road.
Besides, HPI is not good as the parts is always out of stock.
Although I like Kyosho cars very much, I want to give some opinions objectively. I played many Kyosho RTR/basic level cars like Inferno ST, TR-15, FW05, Vone S, Inferno GT.......almost all need buying a lot of extra parts when you change to other brands engine; and the parts/design/materials are really not that good, especially for the off-road ones.
However, for top-of-the-level racing, Kyosho cars are undoubtedly great.
You can say it is cheap and attractive to start GP palying with Kyosho, but I can tell you may find Taiwanese brands may be better for its materials and durability.