Darren:
If I could make a suggestion here....it sounds like you might be best going for one of the Aerus knockoff uprights, that use the U bags....such as ProTeam for instance. These can be found on Ebay quite frequently, usually for around $300-$450 dollars. Since the year 2000, Aerus has put much better motors in their uprights than before, & they do produce quite a bit more suction & airflow. The new knockoffs, such as ProTeam, Sanitaire, HD2, & others, also have these high-powered motors that produce the same suction & airflow ratings the Aerus uprights are capable of. Quality should not be a concern, either....most of these source their motors from the same place, & use common parts....just a different name & color, that's all. And I would expect to pay about half for a ProTeam on Ebay as you would for a base model upright from Aerus.
As far as putting the new-style brushroll in an older Aerus upright, such as an Epic 3500, YES you will be putting more strain on the motor than the manufacturer intented, & it will turn the brush slower. Again, Aerus has made the brushroll motor more powerful in recent years, they had to put more power into it when the new brushroll was brought out into the new uprights & canisters. The new knockoff makes, such as ProTeam, also have the new motor & brushroll design.
Unless you absolutely HAVE to have the electric hose & mini powerhead, you would be best served going with the ProTeam upright. It has one advantage over the other competitors....it's the only one, that I know of, that has a HEPA filter built-in like the Aerus Guardian upright has. And the suction is VERY powerful on it, too....you should have no problem using a Wessell Werk hand turbobrush with the ProTeam. If having the HEPA filter from the Aerus Guardian & electric hose & mini powerhead is a priority, though, I would strongly suggest you look at picking up a Electrolux Discovery Series off Ebay. I think if you got one for under $150, put the HEPA filter housing onto it, bought a hose & mini PN, & budget money for a replacement motor, brushroll motor & brushroll, & you can do the labor yourself I would think you could get away with spending $550-$600 dollars max, & save at least $400 dollars off the price of buying the Aerus Guardian new.
Going through bags quickly shouldn't be an issue. The bags are a decent size, & pet hair has a tendency to pack down inside a bag. It would especially do this in the Style U bags due to their design & placement in the vacuum.
Good luck with your new upright....Rob
