Likely you would not buy a used car because it was both used and a car. You also would not likely want to buy the cheapest used car (at least you would not if you wanted a reliable used car).
When you buy the services of an SEO you are investing and taking a chance. Most successful businesses take chances though most do not take blind risks.
Many SEOs may not have the time to answer all of the questions listed above (as I probably would not answer all of them). They should have time to answer a few of your questions though. It is their job to sell you their services. It is not your job to blindly buy them. In addition to the above questions you may also want to ask a few questions that are not listed. You should ask multiple SEO firms various questions you consider vitally important to your business model.
There is nothing that is concrete about SEO. The things that work today may not work well tomorrow. The people who do the best job of answering a few of your questions prior to you hiring them stand a good chance to also provide the best customer service after you hire them.
SEO forums have a ton of good information and a ton of misinformation in them. Read through a few different ones to learn the tone of each and ask a few questions about things that concern you.
There are usually no guarantees with SEO, but you stand a far greater chance of finding a good, honest SEO if you ask around and learn a small amount about SEO vice picking an SEO firm at random.
Best of luck with your site and SEO.