Hello Lina,
Welcome to the Forum.
Lots of questions, so lots of reading for you to do !
I am sure you will get lots of answers, but mainly the answers are down to your own preferences.
I suggest you consider carefully the use to which you will put your van, as that will allow you to make difficult decisions more easily.
Budget is key. Conversions are not cheap, and a pop top will add considerably to the cost.
My quick summary to your questions.
Rock and Roll beds tend to be narrow, although wider ones are becoming available.
Do you intend to buy a van and have it converted, or buy one already converted?
With a Pop Top consider your ability and willingness to get up into the top.
Pop Tops tend to lift at the front which is not very useful with a rear kitchen.
Coastal car parks (at least where I live) often have height restrictions; a pop top can prevent your access, a high top certainly will.
4wd, depends on your intended use. Imho, not needed; just research your grass pitch campsites well and pray for good weather.
My configuration is V6, no pop top and with a rear kitchen; so I do have a bias.
Others will have an equally valid bias in favour of side kitchens, etc., etc.
Go and inspect a lot of vehicles with different configurations. try not to buy the first one you see !
Good Luck.
As an afterthought. Investigate where you will have the vehicle serviced.
Most official Toyota dealers won't service it as it is an import ! But some will, so it's worth checking your local Toyota dealer.