My first trip was for 8 nights and I'm doing 8 nights in May. I'm lucky - I'm a dvc member now, which helps a lot. I wouldn't do lots of other stuff tho - i will mainly be doing disney and relaxing at the pools and stuff.
We economise by getting an annual pass - not an option for you now maybe - but in the future. For me it is good, because i a getting into the parks free on this trip - paid for the passes last year. If you are mostly just doing disney why not stay on site in one of the values? U could always also add the dining plan, if you eat at least one nice sit down meal per year it's worth it. Do you really need a car? If onsite, you can get the magical express for free, or a town car for cheap. Also, if you would prefer a villa have you considered renting points from a dvc member? Check out the dvc rent/trade boards on this site.
It really is a personal decision - i never understand the people who go for two weeks then cram every single tourist attraction into that time, instead of taking time to chill, but that's their choice.
If you didn't go for a week - what would you do instead and how much would you spend - that's definitely a factor to consider. Also, if you went for 1 week this year - would that prevent you from having a longer trip next year? Only you can decide.