The + or the - are dependant on the type of DVD writer you buy. Some drives will accept - and some will accept + and some of the more expensive will accept both. You will have to look at the spec on the box befor you decide to buy.
Neiro will recognise what king of disk you install and offer options accordingly.
They are expensive. I bought mine from internet company "Dabs".
You can put larger films onto a single layer disk, you just have to compress them more.