I'll point out some of the key features of the Classic Guitar and how those features are important to Classical Guitar playing. The Classic Guitar is constructed to allow the top of the instrument to vibrate but to suppress vibration in the neck, back, or sides. This is similar to how an electric speaker works, the center section moves to radiate the music, the casing must be rigid to support smooth motion of the center. A solid body guitar does not resonate and will not project any reasonable level of sound. The type of string is critical to both the Classic Guitar itself, and to playing Classical Guitar.
Steel strings have a much higher tension than nylon strings when tuned to the correct pitch.
A guitar designed for steel strings will not respond properly with nylon strings, and a guitar designed for nylon strings will probably break in half if you try to string it to pitch with steel strings. Playing Classical Guitar requires that you use a combination of contact with your finger nails and with the fleshy part of your finger tips for picking. The quality of the sound is also very dependent on the construction of the top of the guitar. Famous Classic Guitar makers usually have carefully selected pieces of wood which will eventually be made into the tops of Classic Guitars aging for years in their workshops. Never accept a Classic Guitar with a laminated top, you will be very disappointed with the sound.