1. How much typically will a bare metal respray cost?
Probably $15k to $20k AUD
2. Can one buy 2nd hand Garmin 1000 glass panels and if so how muc do they typically cost? (I want to propose we rather do this).
Would probably cost more than the plane is worth.
There are now better and cheaper options from Garmin and other vendors like Aspen.
3. If we repaint would it be crazy to suggest 'hivis yellow' as it will make the aircraft very visible and improve safety - I will rather have a very visible aircraft than a beautiful colour scheme that is less visible!!
Stick with the factory paint scheme. Looks the best and will have a better resale value. White is a practical colour for planes.
4. Can a ballistice parachute be retrofitted to a PA28 and if so how much roughly?
Close to impossible and not a good idea.
In general, if it is an older PA28 it's not worth spending a heap of money on upgrades. Just maintain it well and enjoy the flying. look for less expensive avionics upgrades.