We're in the same situation looking for a Cayenne S in the early 2003-06 Series 1, or 2008-10 Series 2 - in low miles & well cared for. So I've done a bunch of research & talked to many Cayenne owners who tow, many within our local Porsche club.
My preference is for the normally aspirated "basic" V8 S model which will work less hard than a V6 gas powered one, and with few extras to complicate repairs later, so I don't really care if it has GTS, Turbo, Air Suspension, etc.
The V6 Diesel will tow fine, but the V6 gas will work harder & therefore use more gas to tow, but saves gas when not towing - but not by much. Unfortunately the USA didn't get the Deisel until 2013 - so they're all still very new & pricey, and for the $50-60,000+ I see them selling for now, I can buy a lot of premium gas for the extra $20-40,000 difference between a nice 03-06 in the $10-25,000 or $25-35,000 for an `08-10 one!
Repair & maintenance will be at Porsche prices - so not as cheap as some other SUVs, but not really that much more than say Toyota or other prices nowadays.
The series 1 Cayenne S V8 needs to have several things done to even consider it: the plastic engine coolant pipes on the engine need to be updated to the metal type (series 2 changed this from the factory), the ignition coils/sparkplugs needs to be updated, the Kardan Shaft bearings (drive shaft) need to be checked & replaced if needed, and the front upper ball joints apparently can wear out around 40-60,000 miles. Both series 1, 2 & 3 should have had all required maintenance & fluids changed as factory recommended with factory specified parts by a qualified technition (or better/more often). The seller should have detailed records of all of this, as well as to verify the mileage on the vehicle is as claimed.
You're own Canadian "RV Lifestyle" magazine has done a couple of test tow with Cayenne articles under their "Hitch Hints" section, and the writer runs a trailer & tow vehicle preparation center in Ontario - between you in QB & Toronto. Look the articles up online to read them, and maybe give a call to them for some hitch issues & how it worked for them.
Also check with your local Porsche club for other Cayenne owners, and at some local independent Porsche repair shops thru the club for some feedback & what to look for, as well as to check over any Cayenne you seriously consider buying. If they're a good well recommended shop, they'll probably end up working on it for you.
Tom (the husband