Don't know what birth year or age you are or what level of play you have been playing at, but it it entirely possible to make the conversion to goalie later in your playing time. It just takes more work.
What you have going for you
1) Knowledge of the game gained through experience. Kids who start young both have to learn the game and the position as they develop. You can focus more on learning goaltending now and the theory will come fast.
2) Some experience in roller hockey goal. Clearly you have gotten the bug and don't have the fear issue that many wannabe goalies (wanna be the center of attention only goalies) develop during their first experience. I'm going to discount that experience a bit as from what my goalie D tells me, roller and ice goaltending are rather different animals. Her opinion is that mobility is a bigger factor in ice hockey where glove speed and reach are more important to roller hockey. BTW, she hates roller goaltending because wheels take away the best part of her mobility (lateral).
3) The bug. Motivation is a big factor in success. From your willingness to buy your own equipment, I can tell that you are willing to invest a bit. You do have a certain degree of uncertainty about the butterfly technique (it is not a natural thing nor good for the knees). Goaltending isn't entirely about butterfly saves. The best goalies use many techniques and select the best technique for the particulars of the situation.
I'll tell you that where I live, we had a kid take up goaltending in his 2nd year of Bantam hockey a few years back and was similarly motivated and ended up an excellent HS goalie. His first year was kinda rough, but he had a coach willing to work with him. With some roller experience, you are already ahead of him.
As to what you should be doing right now...
You hit on an important thing about stretching. The most aggravating injury a goalie can have is a groin pull because they are slow healing, difficult to tell when they are healed, and quick to re-injure if you don't wait long enough. And a goalie without good flexibility (developed through a good stretching regemim) will eventually succumb to groin injuries.
Seeing that you are from PA, I would guess that your local organization probably has some type of goalie coaching (clinics or practices) available.
I would try to find out NOW if you can beg into these sessions even though you don't play for a team right now. With your lack of experience, you might want to attend with younger goalies (squirt/peewee) where they work more on basics that you need to practice(shuffling, telescoping, basic butterfly, finding angles, getting up and down, etc.). I think a willingness to learn with the younger kids will demonstrate an maturity and respect for learning that will get you the respect of coaches.
As you quickly master the skills, I'm sure they will bump you up to your own age level quickly.
With a couple months of basics, you will be better prepared for Spring League where nobody plays defense and you will get your resolve tested thouroughly and see lots of rubber.
Plan on attending a couple of weeks of goalie camp this summer. I can't make a local recommendation for you, but look for camps that are run by the principle participants. Many camps are franchised (think Rick Heinz) and can have great variability in the instructors and the teaching. Perhaps Fangers has some other good suggestions in this area.
As to your build, you seem to be good sized, but it has nothing to do with technique. There are big and small butterfly tenders as well as big and small floppers and stand-up guys. The best goalies can do all of the above which keeps the shooters guessing. You will develop techniques that you are more comfortable with, but don't let that limit you as shooters learn how to game goalies. I'm sure you hear about which ones are weak down low or 5-hole or someplace else.
Hope I've been helpful. I praise your willingness to make the change at your age. It takes plenty of work, but if goaltending is your thing, there is nothing else like it.
Feel free to ask more questions.
Been there, done that. Went somewhere else and did it again.