I'm not sure how you can buy a 2003 Sienna for only 7000.00??
I would be VERY cautious if one is being sold for that amount.
Siennas are very good cars but you'll want to make very sure it's been well taken care of. V-6 Toyota engines require strict oil changes. If this happens, they will last 200,000 miles and more. These are prone to engine sludge if the previous owner skimped on oil changes.
Transmissions are fine.