As Mat said, the exhaust will fit but will end 3" behind the bumper, it will be OK of you buy a US spec bumper like the one you link to. The catback section doesn't have the lambda sensor lug cause it is on the cat section on the IS300.
2 choices : weld a lug on the pipe or buy a IS300 cat replacement pipe (Megan one is around $140)
or you can install the muffler alone and try to sell the cat back pipe on eBay but the muffler will not be well placed in the bumper hole as the whole exhaust bending is slightly different from the standard one.