Extra storage

You can use it for storage, but if he plays any Series S/X games, they can only run on the internal hard drive or the Seagate expansion card. So if he stores those games on the external SSD, it will tell you they have to be moved to the internal before the game will run.

yep, proprietary hardware is designed with profits in mind, there is nothing special about them either as most new nvme drives are way faster and hold more for less.

Thank you for clearing this up for me. It’s shocking the price of Seagate expansion c.

[quote user="CooledReesy"]a) will purchasing an xbox 8 - 16 GB USB store my profile allowing me to switch it to a better machine if i choose to buy one?[/quote]Not sure what you mean by "better". The point of a console platform is that all consoles perform
in the same manner over the lifespan of the platform. USB flash drives allow one to move content from one console to another. Whether or not a game will allow this console hopping is up to the developer. Turn 10 is somewhat notorious for locking their Forza
games down to a single console.

b) do i need to bother buying a new machine? will a 16GB USB simply enlarge my storage space?

You could just add a HDD to your existing 360 S console. No need to buy a new console unless you want a longer warranty
on the HDD (HDDs on HDD-based console kits are covered by the console warranty)

c) as battlefield 3 will not allow me to access the High Definition gaming on my 4GB machine, will a USB allow me get this, or will i need a new machine?

You'll need
a HDD. New console is up to you.

To summarise, will a XBOX USB solve my problems or will I need a new 250GB machine?

There are both 250GB and 320GB accessory HDDs available for the 4GB console. There's a trap door on the bottom/right of the console.
That's where the HDD goes.

I would suggest the 250GB HD as did NEO XBOX360.