First and foremost: Thank you for your service!
Secondly: Welcome to the forum! There is a lot of helpful information here, so use it to your benefit!
Here are my suggestions for buying a car:
1) LEARN, LEARN, LEARN. Be sure to understand how to decode VIN and trim tags. Understand where the body VIN derivative stampings are located, as well as where the stamps are located on the engine and transmission. Galen Govier has a small "Chrysler Corporation Production Option Code Book" for 1969 - 1971 that is very helpful.
2) Decide what you want. A "number's matching" car will cost more than one with a transplanted engine. Either is OK, it just depends on what you like. Also, a lot of people take, for example, a regular Satellite and make it into a Road Runner clone (or "tribute"). In my opinion, there is absolutely nothing wrong with this, as long as the car isn't advertised as anything other than that. These should cost less than a real Road Runner, so it is up to you to decide what type of car you are going for.
3) Because real Chargers, Road Runners, etc. are starting to go for good money, be aware that there are fakes out there. If a car is missing its original engine or trans, that's fine. But if the VIN has been tampered with, that's fraud. Learn what the proper rosette rivets look like. They are reproduced, so a good fake might still use them, but they are at least one signal. The fakes out there make it a good reason to LEARN before you buy.
4) It goes without saying that these cars are prone to rust issues. Lean how to spot the rust, and understand what rust is hard/expensive to fix, and what is more "normal" and easier to fix.
5) If you are not super knowledgeable about the cars, another option is to bring a friend or a hired inspector with you. I highly recommend this even if you are very knowledgeable since a neutral 3rd party can help you walk away from a deal that isn't right for you.
I'm sure there are other things that I forgot here, so others may chime in and add to it. Either way, best of luck in your search for the right car!
Hawk