I'd start even earlier.
Is DVC right for you?
1. Do your travel patterns match those of timeshare usage - you will use DVC points at DVC resorts at least every other year. (In theory, you can go every three years, but its tougher to make work).
2. Your planning patterns fit DVC. You can book ahead by at least seven months.
3. You are happy to stay at the resort you bought into. While its possible to book another resort subject to availability, you may be disappointed in the amount of availability.
4. You can work with and understand DVC cancellation policies, point expiration and banking and borrowing. Cancelling DVC is rough and the situations can get complex (if you are further than 31 days out, the points go back in your account, but if you are past your banking window, they still need to be used that use year. If you borrowed any points, you can't return them to the next year. So if you have an April use year and you cancel a January trip in December more than 30 days out, you really only have December, January, Feb and March to use those points. If your availability to get vacation time and DVC availability on short notice don't match - bye bye points. And if you are less than 30 days out - good luck.)
5. You are happy with a timeshare and not a hotel. That's things like you'll have one bed and a sleeper sofa in a studio and no daily housekeeping. Checkin isn't until 4pm (you might get it earlier - you might get in later) and there is no late checkout. You will not be eligible while staying on points for deals like "free dining"
6. You can afford DVC - its a lot of money to tie up in a luxury purchase - and the additional expenses of regular Disney trips can get expensive. While Disney will try and sell you DVC by saying you can save money, many owners here believe they spend more, but get more value out of it. Disney should be fun, not a financial burden. And when the bills are coming in for college tuition or daycare or a sixteen year old's car insurance or needing a new car, it can be a financial burden.
7. You are willing to spend more than you'd spend at a value or offsite. DVC is not the cheapest way to do Disney.
8. You've evaluated owning vs. renting DVC points and deciding that owning is a better plan for you.
9. You are not buying to use your points to cruise or do ABD trips or trade out - those options tend to be expensive and you are generally better off paying cash.
There is probably more, but the first step to a successful evaluation is figuring out if its right for you. There is really a narrow band of people its perfect for - a wider band that it works for, but it certainly isn't the right fit for everyone - even rabid WDW fans.