vRS petrol valve stem seals. What should it cost to replace?

I've recently been told that the valve stem seals in my TFSI DSG vRS need replacing (2012, 140k). It's been drinking oil for a while, rough idles when cold and wet, and when investigated there was oil on two spark plugs and white tips on the others. It currently runs fine most of the time, but will now intermittently throw a wobbly for a few minutes which results in a flashing engine management light, some misfires and kangarooing down the road (hence the investigation work).

I've been quoted £1.5-2k to get it done. My guess is the car is worth £2-3k when running properly, but I'm happy to be guided. I've recently had £400 work done (new spark plugs and PCV) and fitted a new (Bosch Silver) battery.

The car is otherwise in good condition with a decent service history (I think full, but would need to check) and we were thinking of selling before this started.

Two questions:

1. Does anyone have an idea how much valve stem seal replacement should cost? I know it involves removing the head, which is my guess where a lot of the cost is.

2. And is it worth doing to sell as a roadworthy car, or sell before the work with full disclosure? I have an invoice with work done (above) and their suspicions of what the root cause is.

Thanks