Sure, I can offer some tips.
Buying an AED means regularly changing the batteries and testing the unit. You can’t just forget it is there and then expect it to work when you need it.
Using an AED in a crisis can be harrowing, so read the directions ahead of time, and maybe even practice setting it up.
The new ones are really nice. You just hook them up and turn them on. If the patient needs to be shocked, the unit will shock the patient. There is no guesswork, but trying to read the instructions while someone is suffering a heart attack is no bueno.
As for which model or brand you should buy, find out what the police in your area carry in their cruisers and ask whether they like it.
Lastly, if you’re going to carry an AED in your car, and you’re willing to use it on a stranger, go ahead and get certified in CPR and first aid, because the AED is a small part of the overall picture.
Don’t be disappointed if you never get a chance to use your AED, and if you do, don’t be surprised if it doesn’t save the patient. By the time someone needs CPR or an AED, that person is basically dead already or on death’s door, and you’re trying to bring them back to life. If they’re breathing and they have a pulse, they don’t need CPR, and if you hook up the AED, it might or might not actually shock the patient, depending on what the patient needs. Only if you tested the unit recently can you trust that the patient isn’t being shocked for the right reason.