Welcome along
I went from a Focus 1.6 around 100 to the GTI, and it is a big difference, both in terms of interior, equipment and power and flexibility to gain speed in any gear. I have DSG, Leather, Xenons on an 05 picked up with 27k in jan for 13k so you could get everything you want with less miles in your price range.
As for how quickly you will get used to it will depend on what other cars youve driven how much experience you have driving and your driving style, but as long as you dont floor it all the time you should be zipping around without issue straight away its a really easy car to drive.
As for the iphone question it depends what ipod dock you have if its the older version, the ipod plugs directly into the bottom of the unit then no, as this has very limited ipod playing ability, it only really acts like a multi-changer playing the first 5 playlists on the ipod, and a random song selection for 6. If you get an MDI (which wont have been standard fit on any car apart from very new MK5's then you can play tracks off the iphone. This last option will only be available via one of the touch-screen stereos so if it doesnt have that then its safe to say it wont work.
HTH
