Cookster,
how long is a piece of string.
Okay, for what it's worth, here's my answer. Don't spend a fortune on all the grades of a fantastic make of abrasive. I have 120 - 320 painters Yellow abrasive at a few £s per 5m roll. Then I have 400 - 600 in the good stuff. The theory is, the first abrasive removes the toolmarks, the next abrasive the scratches from the previous paper etc.
So only the final couple of abrasives will have anything to do with your finish.
Take a look at the AWGB webpage and follow the Corporate members tab to see which companies offer us a discount on abrasives.
As for a finish on Olive, it's an oily wood, so only an oil finish will really work with it. I seem to remember olive oil can go rancid, so look for a drying oil, like Finishing oil or Danish oil from Chestnut or Liberon etc., Unless it will be for food use, then Food Safe Oil from Chestnut is recommended.
But you might try bee's wax as well.