With your low flying time, the 300 MAY be ok for a 3rd plane and would look very good for a 4th plane. I feel the plane will be too sensitive for a person with limited experience/flying time.

Extra's, Caps, Staudachers, etc are VERY aerobatic planes easiy capable of 3D. They are VERY easy to tip stall and (depending on the manufacturer of the kit/arf ) will snap if you blink hard.

For a 2nd plane I would suggest something along the lines of a 4*, Tiger II, World Models Rambler 30 or 45 (46 size ARF, but very well made ). The Tiger II can be set as a tri gear or tail dragger, the others are taildraggers.

A 2nd plane is USUALLY a low wing, tail dragger. They are not as forgiving as a trainer, but a lot more forgiving than an Extra. With a 2nd plane you will be getting used to a taildragger, more sensitivity, faster landing speed and a "point and go" type plane ALL AT THE SAME TIME. That is quite a bit all together.

Depending on how well you progress with one of the planes I mentioned, you SHOULD be able to handle the Chipmunk for a 3rd plane.

I hate to discourage you, but I would hate to see you waste your money also. Talk to some of the instructors and more experienced people at your field and see what they say. It is extremely hard to judge a person's flying skills without seeing them.