On your budget and you seem to have some building skills.... I'd ask around for someones crashed plane that they don't want to rebuild. It might be more pot luck of what you come up with, but that's ok as it will give some experience with variety though you should stay within your category. A stick for example is a nice second plane but they don't knife edge well or 3-D so respectfully turn down something not withing your interest area. If fortunate, one within your suiting will pop up. The secret is to show a strong interest and those planes that guys have set back to maybe do the repairs on someday, will come out of the woodwork to help a young pilot in the hobby. After you've shown some commitment, post a request on the field bulletin board. Most fliers have one or more of them lying around.

Generally, teen age pilots progress very fast and I'd certainly not say a 3D plane isn't within the possibilities of a second plane though some of us that have been flying for 35 years might have trouble with one.