I'm curious why the aversion to placing it in the attic? No matter where it lives, there will still need to be a fairly tall vertical run somewhere in the mix to get up to the ceiling. I'm not against building a small overhang on the back of the shop and put the unit outside but that just adds to the noise outside for when I'm working late in the evening which could be a point of contention. At least in the attic the noise is contained inside while still not being in the shop.
I would also suggest avoiding the placement of your DC in an attic space if this is a typical enclosed attic space (versus an attic that is wide open to the room below, which would be fine). Attic temperatures in summer are frequently in the range of 120-140F (49-60C, or about the same as some vehicles) and such temperatures, particularly when combined with elevated operating temperatures while running, will dramatically reduce the service life of both the starting and runtime capacitors as well as the motor. Capacitor life is typically considered halved for every 10C (18F) over 25C (77F) and many cheaper capacitors will have a maximum rated operating temperature of around 85C, which sounds like a lot, but in a hot attic with a motor that has been running for awhile that is not much margin and as you approach the rated temperature of a capacitor the expected lifetime can be as little as 1,000 hours, or less, MTBF (for perspective, there are about 8760 hours in a year). The dramatically elevated temperatures are not especially ideal for the motor's insulation nor for maximum bearing life either, but the harmful effects will be most dramatic on the life of your capacitors.
If you are running any DC without its exhaust filter then you will need to initially monitor the amperage draw of your motor during initial installation and ensure that your minimum length intake (length to first blast gate or separator) and the expected length of your exhaust provide adequate intake and exhaust restriction to protect the motor from overload as removing the exhaust restriction can greatly increase the load on the motor if the system configuration allows for too much airflow (the more air the impeller moves the greater the amp draw will be on the motor). If you will be operating the motor in a hot environment (like your attic) then you will need to further restrict intake and exhaust to keep the amperage well below the motor's nameplate specifications to avoid overheating on hot days. If using an inline separator (a la Thien) then consider what may happen if the DC is accidentally run with the separator incompletely sealed, which will reduce intake resistance, and ensure the motor can not overload just because the separator is being bypassed in part or whole. Such are not usually concerns of major significance when using a premanufactured DC solution as there is usually some length of hose or ducting on the intake and there is ordinarily considerable resistance on the exhaust in the form of an air filtration system, but they become concerns when those restrictions are suddenly removed from the system necessitating extra care in ensuring your design keeps the motor within its safe operating region.
I would stay away from corrugated flexible tubing (drainage pipe) and use proper smooth dust collection hose instead as the turbulence in corrugated tubing is extraordinary and greatly reduces efficiency (corrugated tubing is cheap but not worth the savings). Flexible DC hose is not as good as rigid piping, but it is vastly better than corrugated tubing. The DC hose costs a bit more but it is well worth while if you do not wish to further cripple your DC setup just to save a few dollars.
Where you can, assuming a 2HP, or larger, DC, run at least 6" (and possibly larger for larger DCs) for your main runs to the DC, then drop down to 4" (rigid or hose) within the last couple feet just before the machine. For a smaller DC consider 5" and 4" lines. This, of course, assumes you are installing rigid lines. If you are only hooking up one or two tools at a time and moving a hose around then use the best quality smooth walled DC hose you can get and then try to minimize the length of the hose so that there is as little excess length in your hose as practical as the smother and straighter the path of the hose (internally) the more airflow you will have and the better your compromise dust collection will be -- an excessively long length of hose greatly increases losses as the interior becomes increasingly corrugated or the air passes through unnecessary twists and turns.
I wish you all the best in getting your DC setup to fit your shop needs.
