Basically you need a design station where a user can create his own design by pulling and dragging objects, images, adding colors and text.
I am already developing this kind of application where a user will be able to print his own T shirt and system will calculate its price according to his design.
I also can develop a template design station for you where admin will be able to add, edit, delte templates, objects, colors, images etc. On front end a user will be able to select a template and create his own design.
Please send me your details at firstname.lastname@example.org
and we can discuss about it