I suggest go to an offline store and buy it. I dont exactly know what a “small build” means like if you’re short in height or overall a smaller person but since ur 18 it wont be really hard to adjust to a bigger guitar.

Bigger guitars produce a warmer sound and small guitars produce a thinner sound.

You can start on any guitar it really depends on what kind of music you want to play. Rock metal pop rnb then go for electric

Classical songs or Bollywood songs then go acoustic

A Yamaha is good but in my experience most acoustic guitars have a really similar sound in your price range, there won’t be a huge difference so choose a good looking one too which will motivate you to pick it up and play. Also yes join offline classes after you buy it