There are certainly places in Ghana where one would want at least a high-clearance vehicle, with 4 wheel drive necessary during or following rain. But in normal use you're going to be scooting around Accra in traffic, right? Plus weekend jaunts up into the hills or down the coast? Under these circumstances, small and inconspicuous makes sense to me. Unless you're planning to be out exploring back roads a lot, you can always rent (usually with a driver included) on the rare occasions when you need 4 wheel drive.
So the question is, are you really going to be out there a lot, or is this just the way you like to think of yourself--boldly pushing the boundaries of the known in deepest Africa? Will you be driving to Timbuktu? DRC? Even off-roading in the Volta region or chasing elephants in Mole? I'm no expert, and I've been down some tracks in Ghana (and neighboring countries) which would definitely require 4 wheel drive if they were at all wet. But I tend not to go there during the wet. Why would I?
Toyotas are highly regarded and plentiful in Ghana, as elsewhere. I've got no idea how bringing your own would work, but I bet it would be fraught with paperwork hassles and delays. Note that Ghana still requires a carnet, as far as I know, which suggests high import duties and more bureaucracy. They're good at bureaucracy in Ghana, and highly accomplished at dragging it out endlessly. You'll want a local fixer, and you'll want one you trust (a small subset).
One last thought: whoever is hiring you probably knows the answers to these questions (assuming it's a business, the UN or a large NGO--otherwise you'd probably not be thinking of supporting a car). Ask them for some support.
Hope that helps in some way shape or form.
Mark
Edit to add: have you been to Accra? Maybe you don't want a car at all, once you've experienced driving there. This is really worth considering.
If you've got no regard for your health and safety you might consider a motorcycle (which is what I use)--rush hour traffic becomes a minor impediment at worst, an exciting challenge at best. But in objective terms this approach is generally considered unwise.
