Soaking. For the rainfall.... To simulate it, put the garden hose on your tent for a while.
I suggest that you get one that has a bit of a step between the floor and where the tent door zips over as flooding does occur regularly. Be smart and dont put your tent up at the bottom of a slope. If you do have to put it on a slope, put it so the door is facing downhill (water wont flow into the tent but you can face a bit of a problem if it starts to flood back up the slope.)
You will have things get damp. You will end up with sand all through your tent. You will end up with ants through your tent. But you get used to it!
As for the tickets issue. Well its up to you really. You can say you are intending on getting upto Kasane and travelling across the border by road. But some times depending on the customs agent and how they are feeling that day, they may or may not believe you.
You can do it without a return ticket, but thats a risk you have to take. Just be ready with the funds available to buy a ticket.
Just remember, they arent totally stupid. They know what goes on at this time of the year and young guys travelling alone asking for a 90 day visa with no return ticket does end up begging the question.