Welcome to the dark side, young master.
Really, you already have the addiction since you have hardly started down this road and you already have plans to buy another. A friendly word of warning, this addiction can spiral out of control. My brother has over 40 bikes (and growing) and is just completing a 3000 sq ft shed to keep them all in. However, if you keep your addiction in check, it can be a very rewarding hobby.
I, myself, have 13 bikes now and since the only vintage scene around here is in my garage, I race them against the modern bikes. Being that you are recovering from injuries, you'll appreciate having that excuse (riding old bikes) when you are getting beat by kids on new bikes, and not that you are old and slow. I find that I enjoy racing much more even when I don't win because I did it at such a disadvantage. When I get on a friends modern bike, I tend to push myself beyond my current physical limits and can see myself back on the couch, unable to move (again).
The '82 YZ125J is an excellent bike to get started with because not only is it relativily cheap but also and excellent rider. Many folks start out with a bike from their youth only to find they hate riding it after all these years and move into EVO bikes. For me, vintage bike #1 was a Jackpiner and everytime I ride it, I win or break it. Vintage bike #2 was a '71 Pursang and I have yet to complete a moto on it. Vintage bike # 14 will probably be something like your little Yamaha!