For your existing product models, I would suggest creating a custom product module (basically a one-to-one to a satchmo product... you can view this i believe under the custom product documentation).
For your templates, your going to need to merge your design into the product detail templates.
For your site structure (if you have categories, etc.) you will either need to modify the urls file, or take advantage of the built in satchmo categories.
I would say your best approach is:
1) Determine if you need a custom product module... most likely the satchmo product is going to have everything you need
2) Get the site up and going with your a test product (if you need a custom product module)
3) Convert your old templates over into satchmo for your product detail / category pages, etc.
Hope that helps!
Dave
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Dave Brown
CEO/Founder
Rampframe.com - Action Sports Network