If you plan on driving for Lyft or Uber and you're looking for a new used car, then you should set out some parameters before you hit the used car dealer. The car won't just be for your personal use, so you need to take into consideration the requirements for the rideshare service you are planning on using. You also want to make sure you choose a car that is comfortable since you are going to be driving around quite a bit.
Sedan or SUV
You should first rule out 2 door coupes, sports cars, trucks, and mini-vans. Most rideshare companies won't accept these. That leaves you with sedans or SUV's. This is where your own personal preference will come into play. If you live in an area that gets tons of snow during the winter (such as Chicago, NYC, or Boston) then you might like a big SUV with 4wd. This will give you a leg up when driving during a snowstorm (you might be able to get a higher price during these times when other drivers are unable to be out on the road).
The two main rideshare programs both want you to have a late model car. So, you need to pass up on those classic 80s sedans, no matter how much you love the look. You should consult with the particular company to see the restrictions. It is also important to check your state requirements. For instance, if you live in New York City and plan on driving, you are going to have to abide by the TLC's conditions, not just the rideshare programs. So make sure to check to see what your local state requirements are.
It's safest to choose a black or silver color, but that is not required. However, you should steer clear of any custom paint jobs. While you might like racing strips down the front hood, the inspector for the rideshare might frown upon it. Think of what a hire car looks like, and then ask yourself if the car your are looking at on the used car lot would fit the description. Black is universally popular, and it's a great choice because unlike something like white, it doesn't show tiny scratches and dings as easily.
No Body Damage
It is absolutely essential that you choose a car without any body damage. The rideshare companies don't want to be associated with low rent looking vehicles. They don't mind used cars, but they do insist that the cars are in good shape. So, go over the vehicle and check to see that there are no major dents or scratches. If you spot some bodywork done with filler (such as bondo) and a crude paint job, ask the car dealer to show you a car that doesn't have any body damage.