I would suggest that you arrange an airport pick-up and transfer. The idea being that someone meet you at the airport in a van, (by the time you get through customs and baggage claim it will be about 500 AM, the driver can then take you to a place for breakfast. I would suggest The Fairview see ( http://www.fairviewkenya.com/index.htm ), they have a great breakfast buffet and it opens at 600 AM this is about the time you would get there from the airport. Additionally you can store luggage at The Fairview if you need to for your flight to the Mara. Your driver would wait for you, when you were ready to be taken to Wilson (I would suggest an 830 AM departure from Fairview) he would take you there. The Fairview is about 10 minutes away but I am allowing more time as morning traffic is horrible in Nairobi on weekdays.
The above would allow you plenty of comfort, safety, and the ability to store baggage if you need to. If you are planning on spending any nights in Nairobi I would also recommend that you stay at the Fairview over any other NBO hotels.
A reliable company to arrange the above for you would be Outdoor Expedition Safaris see ( www.outdoorexpeditionsafaris.com ) they are based in NBO and very reliable. I have used their services for 15 years now. Their normal airport transfer fee is about $35, however if you wanted to have the driver do the things outlined above the charge would probably be in the $55 to $80 range for up to 4 persons.
Another reliable company in NBO for organizing a car and driver is Market Car Hire ( phone Market Car Hire 225797) but you would have to call them directly as I do not believe they have a website, they also would probably charge you a day rental rate for the car / driver (about $100).
Good luck, Also the Wilson airport is pretty low key, you do not have to worry much about "touts" you just do not want to be hanging out anywhere at 500 AM with all you luggage.