Preping for coilovers

[5509433]

Hi,
I recently looked underneath my car to see how and where the shocks were mounted.
I noticed that the mounting brackets and bolts were very rusty. Should I prep these some how before reusing them?

I was wondering if any body knew the size nut and bolts used so I can replace them if and when I get coilovers.
Also, when mounting how do you know how low you have gone? Where should the spring adjuster be positioned before installing?
Is it trial and error getting the height equal on both sides, or is there markers you can go by?
Sorry for all the questions!

Thanks

Lewis