Hi asweeney, I am half-way through the same process myself with a similar size garden.
What I did was burn off with round-up biactive (supposed to kill everything), level with digger, rake out most of the stones, seed. Problems: we seeded just before the dry spell last year, so grass growth was very poor. The weeds of course didn't mind, so we ended up hand weeding huge amounts out of this section, and it looked bobbins for most of the year, but with weed and feed this year it's starting to look good.
The problem with herbicides/weedkillers is that they only kill what's currently growing. You then have to rake or remove the dead weeds before you seed (otherwise you will end up with patches) and in doing so, you will disturb the surface and get more weeds growing. If you are living in an ex-tillage field, you will end up with mountains of weeds, particularly sharlock the first year (the seeds can live dormant in the ground for 40-70 years!). The herbicides also don't kill deep roots that are not above the surface at the time you spray (e.g. nettles, thistles, dandelions, couch grass, docks etc all that well) so they will re-appear.
Rotavating will chop big roots into smaller ones (which will all still grow).
Possibly the best method I've seen (which the neighbour used) was to get a farmer in with a cultivator on the back of his tractor who made a lovely smooth surface and then rake it for days to remove any weed roots and stones that you find.
You might want to do the garden in sections (to avoid killing yourself if nothing else).
Of course, the father in law just started mowing when he built his house xx years ago - but that was in the days before JCBs/track-machines, scruffy builders - eventually the couch grass and the clover is the only thing that survives constant mowing!