In terms of buying a board, I would say get what you really want and what will suit you the best, becuase it will be cheaper for you in the long run if you pay 450 for the board that will be perfect for you and last you many years to come than to spend 200 bucks every year on trying to keep it cheap. So I would say where you already have some board experience and it sounds like you really want to progress a lot, go out and demo some boards and get a feel for what you like or don't like. Once you can figure out some characteristics that you like, look for them in a board, then narrow your search to fir the description of what you like and demo some of those.
Injuries... whew, well, injuries are never fun, how did you fracture your tailbone in the first place? I would say if it is completely healed and as good as new... that you should be ok, I don't think I've ever heard of anyone fracturing their tailbone wakeboarding. But I'm not a doctor so don't take my word on it.
In terms of teaching yourself and trying to progress your own riding, let me tell you ur not alone! I got started on my own and to this day still don't have anyone that I ride with that wakeboards, they're all freidns and family that just happen to like boating. Its difficult to do on your own and it will slow you down a little bit, but I would say the things that help are: 1) A good instructional video. That way you can see the proper form on how to do things correctly so that your not teaching yourself bad habits. Some good instructionals are The Book, Detention, Higher Education. (2) Learn the basics, my thoughts are that if you learn all the surface stuff first and have awesome control of your board then when you get into the air and landing if you happen to get into a hairy situation you'll instinctivley know how to get out of it. Not to mention once you get into w2w stuff you will be able to progress much more rapidly because you'll have all the fundamentals under control. Plus you wont get frustrated as quickly and you'll have more fun. (3) Bring out a camera and have someone take pics of you or even better video, so you can see what you're doing right or wrong. Then you'll be more concsience of it on the water and can work on improving the bad habits and fine tuning the good ones.
Hope that helps! Good luck, but most importantly: HAVE FUN!!!