There's no reason at all why you can't use round tubes, square is easier to work with for a home builder, which is why it's more common.
Cutting round tube can be done with a tube notcher,
http://cgi.ebay.co.uk/WNS-Pipe-Tube-...item3360cc8870 , the "hard" part is keeping the fish mouths parrallel to each other.
Brackets for suspension etc will have to made, as they are not a stock item anywhere, this is something we could do for you if you wished.
Putting panels on is a bit trickier, as you have a smaller contact area for the rivet, so accuracy is paramount.
On the plus side, round tube chassis's look far better
Kind regards
John