First off, I'd buy the base Macbook, you can upgrade it all yourself, probably better spec for cheaper too, I bought the base one, am fitting a 500gb hd tomorrow, and ram, well I personally don't need anymore, but if you wanted some, crucials the place to go
http://www.crucial.com/
With regards to finance, I'm no expert, but I'd say get something which you'll end up paying least on, aka the total end payment for it. But for that you have to look at interest rates and APR? I honestly am no expert
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But I would say pay it back as quick as you can too, companies like it when you borrow, the longer you borrow for the more money they take, so borrow for as short a time as possible.
If you've had an apple before, and you bought it from an apple store, I think you'll agree buying from an apple store is part of the experience, right?