Check out Horizons Unlimited. Its a great site full of useful info for overland travelling. You might even be able to buy a car from another traveller on there to.
Then on FB you've got Panamericana travel forum. Heaps of ppl doing this trip. Im on my motorbike and will be entering Panama in 2 wks to head North.
I have two solo biker friends ahead of me in Central America having a great time. You will have a fantastic experience of freedom.
Often alot of 'dodgy places' are in bus and rail
stations which you will avoid by having your own transport.