Spotify off-line mode on an iPhone that I dont have.....

[TLewisAsh]

Hello

I currently use spotify on my laptop, with a free account. I'm interested in the off-line facility offered with spotify premium, but I dont have a mobile device as such at the moment.
I have a Creative Zen mp3 player, but I presume that when spotify downloads the music to be played offliine, the data is stored in some kind of proprietary format, rather than standard mp3 files, right?

So my plan is to purchase a second hand iPhone from a well-known on-line auction site, and use this as a makeshift personal mp3 player. But what I dont quite understand is how to get the songs from spotify online to the device itself?

So, do I need to actually have a phone contract to go with the iPhone that I buy? Or would it be possible to just have some pay-as-you-go credits so that I can initially get on-line and download the spotify app (is that right?) and then once thats done presumably I can just download the songs from spotify onto my laptop, then transfer them to my iPhone to listen to offline?

Or am I being really niave about how the processs works?

Any advice gratefully recieved.

What I dont really want is to get locked into some expensive iPhone contract, I already have a bog standard mobile phone that suits me fine and the only reason i'm considering buying an iPhone is just for the spotify music library.

Many thanks

Tom