you can get away with not cutting the harness by pulling it through the enlarghed speedo hole( thats how i did it) however you will have to leave the ecu hanging in the glove box because it wont reach to the stock location.
(another option you can do is have an ecu extension harness made, where the plugs wil just plug into your harness and then connect to the car, i have some available if you need them)
the car body plugs that control relays/lights etc will need to be rewired ( its easy but takes a few hours of work)
you can save money on throttle cable by going to the junkyard and finding a v6 camry from the 90s. the throttle cable runs to the front of the engine bay to a cruise control module and its the perfect length for a 1jz
I would go straight for solid mounts, My car tore apart the polyurethane mounts(the one on the exhaust side sits right under the turbos and gets soft). there really isnt that much vibration, you only feel it when youre easing off the clutch