I think that will be a difficult, if not a nearly impossible task.

You ask for "at least" the amount of internal memory available. I would say that you can never get more than the amount of internal memory if you only use internal memory. You probably will need some internal memory for stack usage and possibly for other house-keeping business, unless you are very careful and use only the registers (which by the way are also in internal memory, and you lose those anyway for you buffer).

Too make a long story short:
You cannot use more than available internal memory to define a buffer. You will most likely end up with less than internal memory size as buffer size.

So you will have to redefine your requirements. Use a buffer size that is taking into account the memory needed for other things. Or just use another part with more internal memory if the actual buffer-size is the problem.

regards
Patrick