The pdf was a fun read. Thanks. Don't know where did you get the figure of 35kg though. The article talked about a payload of 12kg if the balloon is filled with NH3. My estimation is that the payload can be increased to about 60kg if hydrogen is used instead, assuming the Mars does have plenty of water. With a bit of more creativity, that figure can be improved even further.
I had at first put the 12kg max from the first figure, but on re-read I saw the water vapor pressure could have a balloon go as heavy as 35kg. It would makes sense that helium (or more preferably hydrogen) would work better. I suppose I should RTFA better next time.
"if Helium is used instead, assuming the Mars does have plenty of water."
I think you mean hydrogen? Assuming we can make a rover that can collect non-trivial amounts of water (say by baking it out of the dirt), we could make a hydrogen filled balloon with electrolysis.