Steve,
As others have stated, you might want to consider subbing this out.
We did a car park earlier this year around the same size, it took 3 days to wash, 2 persons, 1 1/2 days to sand, 3 people.
Now if you go for this alone, some things to consider,
Where is all the sludge going to go, what is the water access like,
Where is the waste water going to go, yesl alot will go down drians but are they clear or bunged up will the car park flood.
You are going to need to prepair H&S docs for this as it is commercial.