suspension and brakes

Pricing will be difficult as we use US numbers and those do not generally match with the UK.

However, it sounds like it needs rear springs, Here those are less than $60.00 for the pair, but I would also replace the rear shocks at the same time as they need to be removed to replace the springs. Those are about $80.00.

It needs rear brakes and rotors, OEM equivalent here are around $100.00 for a kit that includes the pads, rotors and hardware.

The suspension bushing is about $10.00 but depending on the shop it would cost more as it has to be pressed in/out. It may actually be cheaper overall to just replace the entire control arm. That gives you new bushings on both ends and a new ball joint. A complete arm here is around $50.00.

The whistle could be many things, from a serpentine belt that needs replacement (from the description it seems it changes while steering which would fit a loose belt/tensio-ner).

Then you need to figure in the labor to remove and replace these parts. That varies a lot by location.
In the shop here this would run about $700.00 for parts, labor and cover it all, unless the whistle was some strange costly repair.

As for order of repair, I would do rear brakes, then the A-arm, then springs and shocks. The whistle would be a bit of hunting to check what it is and repair it, but as it is an unknown item I would just have a couple places look at it and get some ideas.

I would also inquire about the timing belt and related parts. That engine can destroy itself if that belt fails, so I would try to find out if it had been changed out or if you will need to do it. If there is a doubt that it was changed I would change it out.

Hope that gives you some ideas. As for total cost of ownership. It really depends on how you drive and if anything unknown were to pop up.