I never needed a second person to land my kite or foils
So far, me neither. But in certain conditions, it can be very cumbersome to keep a 2-line-foil from relaunching.
But I have never flown with strong wind (>30km/h).
Flying in 50+ km/h, you might start changing your opinion about certain things. 
if I have a doubt I use a ground anchor
That's pretty much pointless with a 2-line foil.
If both lines are attached to it, it can easily just launch (unless the kite is weighted to the ground).
If only one line is attached it can start looping.
And if there's not enough wind for it to do one of the above, then you don't need a groundstake anyway. 
Use of a groundstake makes sense when using 4-line-foils, 2- and 4-line-stuntkites (the ones with rods), and depowerkites.
for me, flying a kite/foil must be sort of a challenge. I need to stay focused,
That's absolutely fine.
But for some reason you seem to think that flying 4-line-kites are less of a challenge and don't require to stay as focused as with 2-liners. In my opinion this is a misconception.
The reason whether it'll be a challenge or not has more to do with the speed, pull, and character of a kite.
A bigger kite in more wind will give you challenge enough (provided you don't overdo it). Same goes for the speed of a kite.
What I mean to say is that 2 lines don't necessarily give you more of a challenge than 4 lines. Or better: 4 lines don't give you less of challenge than 2-lines.
Both can be a lot of fun. But you can do a lot more with a 4-line-foil in the long run, which means it won't get boring after some time (as quite a lot of flyers have experienced at some point in time) (
And you could fly a 4-liner just like a 2-liner by not making use of the additional lines (unless you want to).
The other way around is more difficult (=> not possible
).
Don't get me wrong, I am not here to convince you. Just wanted to point out something that I feel you have a wrong perception of as you never seem to have flown a 4-line-foil before.
Anyway, you have something that you can take into consideration if you want to.
If not, that's fine as well and I hope you find the kite that suits your needs. 