I'm planning to purchase an AP using the DVC discount. I'll be at WDW early Oct. Can I order my AP and send a check for payment or does it have to be done by cc? Also, is there any advantage to ordering it ahead f time or can I just purchase the AP (w/DVC discount) when I get there?
You can wait until you are there to purchase it. The advantage I know in ordering it early is the ability to purchase the Disney Dining Experience card which gives a 20% discount at most WDW full service restaurants. But you could even arrange to do that after arriving at WDW but would miss some days of discounts since that office is open M-F.
I don't know the answer to your check question but it certainly would seem that you should be able to do so. You have lots of lead time.
Another advantage is you can get into the passholder section of the Disneyworld website with an AP voucher. That allows you to see all the discounts available to passholders (i.e. you can book a tour in advance and get a 15% discount).