Top soil before a lawn - how much?

Having mowed BIL's weeds (can't be classed as a lawn at any stretch of the imagination) today, he is now looking to have the area cleared and re-turfed over the next year or so.

The first thing that needs to happen is for the lot to be hit with glyphosate in the next couple of weeks to kill absolutely everything, and then it will all want rotavating; the soil is pretty rotten to be honest, and it is a bit lower than he is wanting, so a load of top soil will be needed before turf is laid. Can anyone advise how much top soil will be needed? The garden is 18ft wide, 36ft long and I reckon a depth of 4cm will be required for the top soil?

 

Apparently he has had an offer from a guy at work who will provide and lay turf at mates rates, but he needs to prep the ground in advance. I am not convinced that either turf or seeds are a good idea to be honest, I would much rather he laid weed suppressing membrane and then put a load of bark chippings down to make the place look tidy. A gardener, he is not, and I have my doubts if new grass or seeds would see a sprinkler for more than a few days.

Thanks for the info