This will take a long time to answer... I will do my best.

I am going to spray duratec white with the gloss additive   
Can I spray over the colors in the old gel??? 

You can, but you have to be mindful of the colors you are covering, depending on how heavy you want to go over the old gel colors, I would recommend using a tinted Duratec Primer, they make a white you can tint to ensure good gel coverage... Depending on how bright of a white you are looking for, blue will give you the best results... Just enough tint to differentiate between the primer and the gelcoat. You don't want the old colors bleeding through.

Yes I know it will hide them.  Then I can put some graphics on

The proper technique is to put the graphics "in" the gelcoat, the process is a bitch and takes years if practice. For the novice, I recommend doing the white, color sanding the gel and doing the graphics over the gel in acrylic urethane.

How much sanding do I have to do for prep??

You definitely want to block all imperfections out very carefully, the largest issue with going over old repairs and gelcoat is the bleed through of porosity. A good primer and block sanding, followed up by colorsanding will create a fairly perfect surface for which to build on.

Is there something better then the duratec or is it good enough

There are some products that are more user friendly, but the end products are quite the same. Duratec and Hawkeye are basically for mold making and repairing, very durable end product. It really depends on what you are doing and the resources you have to help you. a gantry, a large shop protected from the wind, a kick ass air compressor with a good dryer, time, will all come into play. Spray additive or not, the entire boat will need to be cut and buffed once sprayed to rid of imperfections and orange peel.. The labor involved is quite intense.

How do I fix the stress cracks in the old gel??

Short abbreviated answer. The crack needs to be ground out completely. If the crack penetrates all the way through, the ends of the crack should be drilled (1/8") and fiberglass and resin used to make the necessary repairs. It is always a good idea to try to find and repair the issues that caused the cracks to form. You cannot simply use body filler as it cracked for a reason, it wont be strong enough.

Hope this gets you pointed in the right direction.

GT