What kite to use??

Depends what you want to do, what conditions you ride in, terrain and what kind of buggy.
For years everyone used open cell foils and they are still widely used for buggying. Easy to set-up and there is a wide variety of performance levels in design as well as price. If you want speed a race foil will give it to you but in gusty conditions or moderate skill level the kite will be frustrating. high end race kites are usually twitchy for the average person.
With an open cell foil you will need a number of different sizes unless you have ideal consistent wind. As wind changes you'll always be a bit overpowered or underpowered and will want a different kite size.
You can use an open cell kite on handles and get 100% control of whatever you need to do with that kite - as long as your competent with your skill level otherwise I'd recommend kite killers or don't hook into a harness and remember you can always just let go of the kite rather than getting dragged.
Depowerable foils give you more range of use. You can take a larger kite out, depower it and when running downwind power it up more, and most designs are a bit more user friendly when it comes to gust absorption. You'll kite with a bar which takes a little getting used to going from handles but if you are comfortable with a bar it's a breeze and one handed buggying and park and go is simple.
So with open cell foils you can start cheaper and may need to get more sizes, can still be very expensive with top of the line kites as well.
Depowerable costs more but get away with less kites and don't need to change kite sizes throughout the day as often if in a place where wind changes.
Do you have a small freestyle buggy or bigger heavier race buggy? Are you on grass, hard pack beach, other? Do you want a kite you can use to buggy as well as surf? What's your skill level?
Where are you located maybe I can give you a better idea of what you can expect for where you are.

For my fixed bridle open cell foils I have 1.8, 2.5. 3.5, 4.5, 5.5, 6.5, 7.5, 9 then with Peter Lynn's 8m Syn, 10m Scorp, 12 Syn, 16 Scorp
Even though I don't need all those overlapping sizes on the depowerables, some are older and used as desert kites where they take abuse. When the winds up to use smaller sized depowerables that's a blast. I rarely use my open cell foils as much in the last few years.