Secure motorcycle parking is available at some of the guesthouses in Arusha near the stadium (all are listed in the Lonely Planet), although you would struggle to fit in a 4x4.
Very happy to recommend Justin and Mike (driver & cook) as safari guides. Both are independent, but work with Kilimanjaro Crown Bird Safaris in Arusha. A decent low end camping safari for the Serengeti or Ngorogoro will cost you about US$90 a day.
Make sure you pop into the International Criminal Tribunal for Rwanda, which is based in Arusha, while you wait for your safari. Fascinatingly inept display of how not to run an international court, which costs millions and convicts very few.