It doesn't say what RPM the machine runs at, but if it's designed as a buffer/polisher I reckon it's going to be 400RPM or more, definitely NOT suitable for carpets!! It'll chew them up.
No budget for a Duo??? They're not that much, think of all the labour it'll save you, after a few jobs you'll be glad you got one.
Something like a 15" rotary is a great tool to have, mine travels everywhere with me. It's a great machine used with the brush block for polyprops or any trashed carpet with hair/fluff in it, and bonnets for less minging work