If you're going to write a thesis you'll probably have to prove it works. If you can prove your wireless sensor network thingy works without one of them cards, then you don't need one. If you can't, you'll need some hardware.
My guess would be that it's going to be rather hard to get access to a wireless sensor network without some sort of hardware. Perhaps you could come up with some clever simulation though, but you'll have to find out if that's possible or not, in the early stages. If you do some research on that, and document it properly you could even use that in your thesis.
I know it's real hard to limit the scope. I too am currently working on my thesis, and the possibilities seem endless. Properly defining limits prevents from doing a lot of unneeded work, or just too much work for the time you have to complete it. If you're doing it for some company or university, they might have a budget for the hardware you need. Just make sure you know what you need, and tell them.
Not sure how much use this is to you, but I just thought I should say it.
Good luck,
Jacco
