@ Chaka
About the soil,I may not be able to advice you on the best soil for growing onions but from my experience on my firm,sandy black cotton soil is good and i believe that red soil would be perfect. Onions need loose soil for easy expansion of the bulb!
On the marketing bit,it depends on the volumes produced and the timings for your crop. As i mentioned earlier,I did it on test basis and managed about 4tonnes. I packed the onion in nets and managed to retail them in my local shopping centre here in Nairobi at the rate of about 15-20nets per day. With Large volumes you need to get a broker who will buy the produce from the farm and take it to the market though at wholesale prices! Dont worry about the market,no one can meet the volumes demanded in Nairobi from his firm even for one day and if you want to know that just go to Marikiti from 4.30 in the morning and see the volumes supplied!