For 4, it depends on how much mess you want to make, and if you want to paint it afterwards. Some options which work for me, depending on what the part is:

1/ -Hand scraping the thick gunk then sandblasting gets everything off, but don't try and reuse the sand.
2/ -Hand scraping then aerosol spray on degreaser gets most of the gunk off, but still leaves a film.
3/ -Hand scraping then blast it with kerosene in an air driven kero gun.
4/ -Follow up 2/ or 3/ with a handful of washing machine detergent in warm water and a scrubbing brush. Gloves are a must.

If you want to paint from bare metal then 1/ is the quickest, or 2+4, 3+4, followed by paint stripper and a wire brush.
If the part is rusty, and not too big, I use an electrolysis conversion bath which usually removes the paint as well.

For the bushes, I took the control arms to my local Pedders, they pressed out the old & pressed in the new ones for a quick cashie.

For the ones in the diff itself, I used an air chisel to remove them & then froze the new bushes, made a tool from a bit of pipe & hammered 'em in.

It sounds rough, but it worked for me, like you I had the housing off the car

Thanks all. I got Pedders to remove the old ones from the arms, etc, which I will get sandblasted / powdercoated before putting back in. Managed to get the old ones out of the diff housing by drilling out the rubber, then some good old fashioned hammer and cold chisel work (without damaging the wall of the diff housing. I've got a plan to make up a simple press with all-thread and a large socket to put them back in - I'm sure with a bit of emery paper clean up and touch of grease it will be easier than removing a rusty one.

GM5735 - used half de greaser half turn, pressure washed them to remove the bulk of the grease, then submerged them in hot water and dishwashing liquid for 5 days - another pressure wash and they came up a treat.

Get a copy of the Holden manual if you can. The bushes are very easy to replace with simple home made tools and a couple of spanners.

No need to pay someone for this and many of the pro fittings I've seen using a press have actually wrecked the parts.

All you need to do is make a couple of sleeves out of some steel tube (gal pipe is ideal), get two small pieces of steel plate, a length of all thread that fits through the bush with nuts and washers, and an old front wheel bearing outer shell. The bearing shell is the most important part as it fits the flange on the end of the bush and protects the sleeve and the rubber when you pull the new bush in.

Basically you make a remover/installer tube, a pair of end plates to tighten against and a spacer to go inside the control arms (to stop them collapsing), once assembled, you simply use the spanners and the all thread to pull out the old bush and then reverse the assembly to pull in the new ones. If you have an angle grinder and a drill it will take you no time to make these tools and they will last forever. (These are basically the factory tools.)

If you are a good contortionist you can even change some of the bushes on that car using this method.

PS you can use a similar method in a large bench vice to press in ball joints. You just need to make the tools out of gal pipe, the wheel bearing shell fits the bottom of the ball joints too.