Hi divadobs,
I agree, a garden of your size, terracing would not be the answer. You could re-use the pebbles to help make up the levels, but I would be reluctant to use any form of rubble as this may result in drainage problems. If you have no other alternative, I would suggest that you mix it with as much soil as possible. I would suggest that you allow for at least 6" (15cm) of top soil for the grass to grow in. Make sure you really compact all the made up levels, otherwise you will find after the first winter, you may end up with an uneven surface. If you are going to use turf, they should be lifting it shortly, but of course this is a lot more expensive than seed. If you don't want an instant lawn, then sow grass seed once the soil gets a little bit warmer (in the South this will be around the middle of March)...Which ever you use, make sure the ground is firm...Good luck. I would be interested to know how you get on...If I can be of any further help, please contact me