Hallo bibka,
Welcome to the crowd,
in order to answer your questions it would be of interest to know how long you intend your trip to be, which areas do you want to visit and what kind of accommodations you are looking for.
In my opinion April/May is a good time to travel , March might still be a little bit wet, in June it is becoming cold.
Depending as what your routing will be, you don`t need any malaria prevention at all. In the event of a heart disease it will be safest to consult a doctor in means of malaria prevention.
I am very much in favour of camping, you are so much more into nature this way. At some places I even sleep outside under the sparkling stars. You wrote you are experienced campers, so do`t worry about the roof tent, it is easy to built up and close again, there is nothing to take into account, except, don`t fall off the roof.
I think reservations of campgrounds will not be necessary at this time of the year (yes campgrounds are open around the year), except: Etosha, Sesriem and maybe Fish River Canyon.
Sorry but I do not know the two mentioned tour-operators. If you want a tour specially made up to your wishes, there is www.bwana.de. He has a flyer which is called Reisetraum, just fill it in an send it to him, he will make up the tour of your dreams.
I don´t know what to recomment in means of driving a 4x4 car. I don’t see any difference driving it on roads tarred or gravel, sand is different, but I am not sure whether you will encounter sand pads on your tour.
So I hope that helps a little bit, just keep on asking questions, we will be happy to answer.
By the way, where are you from? GB? Then you at least don’t have the problem of driving on the “wrong” e.g. left side of the road.
Greetings from Lake Chiemsee in Bavaria
Savuti

Hello Bibka

The agent for our travel agencie in Namibia was Sens of Africa and I can highly recommend you Sense of Africa. We only made very good experiences.

I cannot help you with camping as our tour was only in lodges but we very much enjoyed every single lodge during our trip.

Wish you a good planning period and just ask further questions which you may have:-)

Regards
Namibia07

Hello Bibka,
welcome to the Forum

We've travelled twice in May/June. It was really a lovely time of year but gets very cold in the evenings and early mornings. If you're camping you'll need some warm clothes.

If you have a look at the Sense of Africa Website have a look at their Free and Easy vouchers. It's good way to stay in excellent lodges for a fraction of the normal price, so perhaps you can do it like we did and spend a few nights in a nice lodge in between camping. If you look in the search function of this forum you'll find lots of info on the vouchers.

Have fun with your planning

Sue