The setup was done with the rear seats in and I didn't change it as the lower seat squab only weighed 5-6kgs anyway and the battery was very close at 4.8kgs with box and wiring. The rear upright section has always been in the car as it helped the car feel a little less "flexy" in the rear. Cage will now fix that! :) Any small amount of extra rake was welcomed for handling. The only thing really different would be the battery with the stock unit weighing 12kg+ and all hanging out the front of the axle line like its sitting on a lever!!!
The car will be basically different, but my advice get them in the car as is, drive it for a few km's to settle them and then sort the heights from there. First check the front passenger side against the front drivers as this is likely to be the biggest difference. The rears should be even but you might need extra clearance for the 18's so check heights. Really any adjustments should be very small. IMHO, I would not pay $450 to have someone else do it as its really really easy. You just measure from the centre of the hub to the bottom of the guard.
So hot tips:
Check fronts side to side first.
Once the front is done, sitting down drive it back and forth to settle it and steering lock to lock. Then measure again to make sure they match side to side. Fronts have the most variables so do this first and take your time.
When setting the rears, give the car 5 to 8 degrees of forward rake (front down) for handling.
As the springs are not linear rate, there will be a certain amount "give in them and you might be able to do a fit and forget, but once installed settle them in and then measure and adjust.