Wow, there is some crazy info posted in this thread!
First thing to know and worry about is making it through the physical entrance into the armed forces. Many things can disqualify you, your spinal condition being one of them, depending on severity. The Docs there at MEPS are there to disqualify, not qualify you. Stop taking the Propecia, it's not going to be something you are going to be able to stick to through any of your training and the less meds you are taking, the better off you will be for now.
Next order of business is making it through basic training. Don't get so far ahead of yourself that you already have your mind set that you are going through Ranger school. Forget about any "contract" you sign telling you that you are going to be a Ranger. The bottom line is if you don't make it through Ranger school, you're not gonna be a Ranger- period. Take things one step at a time and concentrate on that task. Many soldiers try to become Rangers and many just don't make it for various reasons.
Regarding Rangers that get their tab and don't actually perform duties as Rangers- that's very common as many Soldiers go through Ranger school and don't actually serve with a Ranger Battalion. Even the Marine Corps sends Marines through Ranger school, as billets become available, and obviously they are not serving as Rangers. It's a tab, and has many different applications in your military career. You very well could make it through Ranger school and be a support person at Ranger Bat doing clerical duties, it all comes down to the needs of the Army.
Anyway, you have plenty of time to get physically ready for basic training. Concentrate on core strength, endurance, flexibility, and most importantly keeping your body healthy and strong. If you get the opportunity to go to Ranger school, go for it if you think you can mentally handle the challenge. I mean what's the point in doing something that you know you're not gonna make it through? One step at a time and you'll be fine.
And whoever said they won't care about you taking roids needs to get their head examined. You couldn't pick a worse time right now to be using juice in the military. They are following suit with the rest of the world cranking the screws down on people using. i wouldn't even consider that at this point.
Good luck
Doc M