I almost missed your question. Sorry I didn't respond until now.

checking the fluid color, level, smell is a very good first step to do. You will definitely get an idea if the transmission will need a rebuild. If the color and level is good, then your transmission probably doesn't need rebuild.

When the transmission oil becomes very dark with burnt smell, that is an indication that the clutches are slipping and degrading the oil. When you find significant amount of fine debris in the oil pan or filter, then it is a sign that the transmission needs rebuild.

In the old cars and trucks with automatic transmissions, the transmission's valve body was purely hydraulic. It consisted of a massive maze of channels and valves to direct hydraulic pressure to the pistons which would actuate the different clutches in the transmission. When the transmission would have problems, it would often lead to a rebuild or major repair. Many times requiring major cleaning of the valve body. Due to the complex nature of the valve body, it meant that the transmission would need to be worked on by an experienced transmission shop.

You are in luck. Newer transmissions have electronic solenoids that replace most of the valve body. Instead of a crazy maze of hydraulic passageways, newer cars and trucks use solenoids. Solenoids are electrically controlled valves. Your car's computer controls which of these valves are in one position or the other. The great thing about these valves is that they are usually easily replaceable. It does require removing the oil pan or access cover. Diagnosing the problem to a solenoid can be a little tricky though. There are generally 5-6 solenoid valves in a transmission and they often work in some combination to actuate various speeds of the transmission. Finding the correct solenoid or combination of solenoids that are causing the problem can be tricky.

If you are handy and have a repair manual, one approach is to buy a refurbished set of solenoids and simply replace all the solenoids. Refurbished or aftermarket solenoids are very cost effective over new. It can get very expensive to replace all the solenoids with new OEM parts. If you want new OEM solenoids, then it is better to find which solenoid or combination of solenoids that are causing the problem and only replace that part.

If you don't want to attempt to work on it yourself, then I highly recommend to take your windstar to a transmission shop. A dealership can certainly diagnose and repair your vehicle, but expect to pay much more at a dealership. The benefit of taking your vehicle to a mechanic that specializes in transmission repair is that they will have the necessary diagnostic equipment to find where the problem is. It could very well be the pump, torque converter, actuators, clutches, valve body, or solenoids. They have the ability to diagnose your windstar without removing the transmission from the car. Given that you have good color and level of oil, it is a good chance that they can fix the transmission without removing it.

Again, Sorry for the late response. Good luck getting your windstar back into top condition.