Retainng Wall Car Park

Hi I am wanting to build a retaining wall, 500mm at the highest point, 7m long there will be cars parked at the top so there will be a load on the wall.

Do I need to get a egineer to design it, I was going to have the posts h5 90x90 at 1000crs, would this be ok?

Was told whatever is sticking out of the ground, that should be the depth of the hole, is that about right or should I go slightly deeper consedering it will be used as a car park above.

Also for the drainage, I know to use the draincore pipe and some metal but do the pipes have to lead into some sort of drain or just to be diverted away from the wall?

Would you nail the boards on or bolt them into the posts?

And have aslso seen walls that have been build out of plumb leaning into the bank. Why is this?

thanks in advance to anyone that can help me

I think with car parking near the top you will need an engineer's report.

I think you will have to go far deeper as you are asking an awful lot of the wall.