I recommend against aluminum frames/forks, especially from the used market, because of aluminum's limited fatigue life. After a certain amount of use, it can unexpectedly fail, catastrophically. In contrast, chromoly and other varieties of steel don't do that. Aluminum was introduced by the bicycle industry for regular consumption, with a selling point of it being lighter than "steel" (ambiguous terminology deliberately used). The folding mechanism of an aluminum frame has enough material to negate any supposed weight advantage and this part of the frame is especially susceptible to fatigue. So that's how I compare the Qix D8 with the Speed D7.
I don't have any experience with the roller rack that you're asking about. However, I do recommend exploring the roll-ability of side-folding bicycles such as the Dahon Speed. Mine rolls well with the handlepost folded across the top of the frame (instead of tucked down between the fold). I just pick up one end and roll it like a wheelbarrow. Also, there is a product by Litepro which is
a castor that fastens to the bottom end of the seatpost. I don't know if it works well though. I been rolling my folded bicyce around without anything to facilitate it.