All depends how rough the ground is now. After box-blading I would think it would be fairly smooth. In that case a chain harrow may work very well. However, if it's seriously rutted and clodded, the chain harrow won't do a lot and the disc might be needed. What do you already own besides the box blade?
This past spring I turned 5 acres of weeds into hopefully a new hayfield. I did 1.5 acres with only scarifier teeth and a chain harrow. The other 3.5 I used a disc between those steps. The area that got disked is much smoother, but only because all of it had been scarified first. I don't own a box blade and didn't try using my grader blade. I wouldn't buy a disc for only this purpose because they are quite pricey and may bring up enough weed seeds to offset the smoothing benefit.
As for grass choices - well, Oregon is where most of the US's grass seed is grown. There should be lots of choices. Decide if you want mere groundcover or something to feed wildlife or livestock first, because it will be different seed choices. Then go discuss with your local seed supplier, farmer's co-op, etc. They may have a seed catalog that you can browse and try to guess what will work best. Look up the website for a supplier called "Forage First". I'm guessing a mix of perennial ryegrass and fescue will be a good bet, but it's only a guess. Kentucky Bluegrass may also be good if you basically want a lawn. These three grasses are the most common lawn grasses in areas that don't get super hot or dry. If you want to attract wildlife, you'll want more than just grasses. If you want to pasture horses or cattle, you'll want high yielding grasses, may need high traffic tolerance, and you won't want turf type fescues because of a fungus that grows in them that's bad for most grazing animals.
