I would also look at what you might want in the future as some stuff is hard to retrofit.
Stuff like the iPod connector can I think be fairly easy to add, although the location in the car would probably need to be different.
There is some debate as to whether or not it is legal to retrofit HIDS in the U.K so I would opt for these upfront.
I think the bluetooth is possible although a pain. As said it give much more than just bluetooth.
A few people have retrofitted the sat nav and think that's fairly straight forward.
A dealer can easily do parking sensors too.