The DR30 is an excellent choice for you classic Skyline purchase. You will find that a lot of the Australian delivered R30 parts are interchangeable, so the rear wiper arm and motor might be easier to find than you think.
As for the front seats, when I owned my DR30, I replaced the seats with a set of R34 GTT front seats and had the front and rear retrimmed to match. They were a relatively easy fit and in one afternoon, I had modified the original R34 rails to suit the R30 floor pan.
Because they are such an aged car, I had to do the same tidy up routine as you will and build a new parcel tray and re-carpet and clean up the boot area.
Good luck with the new ownership and happy Skylining!