Personally I would rather buy one without any or few mods. That way when You add your own mods to it, You will know how to repair those mods if something goes wrong. I understand wanting to buy one already tricked out, but Who will be there when something goes wrong.
Have you ever heard the phrase "Built, not bought"? If you build your own, you will have something to be proud of and will be able to hold an intelligent conversation, when people ask you about it and how it was done.