Here is my take on what you should look for.
1. Choosing the car - Look for what you prefer. Do you want a 2 seater or a sedan/coupe. Look at the resale value - what are used ones selling for? Look at how long it takes to sell the car (I expect Z's will take longer than a 3 series - then again the 318 is a tough sell)
2. Unless you are very knowledgeable about cars, don't buy private party or from a used car lot. The reason you pay more for a Certified car is because your local BMW dealer is putting BMW (the manufacturer) on the line. If the dealer blows the certification, the manufacturer pays. A car must pass certain criteria to be certified - no accidents, clean title, no rust repair, mileage limitations, fully serviced (these cars are not cheap to service/repair), tires/brakes are at least 50% or better, etc. You also get an additional year of warranty and mileage warranty increases to 100,000. If you were purchasing a cheap car, you could take a chance on a private party, but $22K is a lot to lose if you get a "bad" car.
Also, consider the reason most people get rid of their old car.
- They just had it in the shop and their mechanic told them it would cost $$$$ to fix. They would rather dump the car and use the repair cost for down payment. This is the category most sellers are in.
- The car is a lease and the term is up. Typically a good car, but you cannot get your hands on this car, unless BMW dealers refuse the car and it goes to auction (if the dealers don't want it, why would you?). These are the cars the BMW dealers keep, certify and resell.
- The seller has grown tired of the car and wants something else. A good used car, but how do you (the buyer) differentiate from the seller who knows something is wrong?
- The seller needs something different (just had another child and has outgrown the car, can't afford the car, etc). Another good car, but again, how do you know the seller isn't trying to dump his problem car on you?
- The car was in an accident, was repaired and the owner has bad feelings or the car is still under warranty, but the dealer has not been able to remedy the problem. These may seem like good cars, but I would avoid them. Cars are not designed to be smashed, then repaired (cars are designed with crumple zones - you cannot repair this to the level the engineer designed it to).
I am an auto broker and I deliver about 375-400 cars per year. I have heard all the stories about how great a trade in is, but when it goes to the dealer that purchases the car, then goes up on the lift everything changes. Most people will keep their old car until it becomes too expensive to fix. This typically happens after they've dumped some money into their car, then it needs more and finally more. They dump the money pit and start fresh with a new car.
Good luck and I hope you listen to the points I've given.