Make sure that the foundations are good as other posters have stated but
also give them enough height say 2 to 3 ft, gives you more space

For sure on the foundations, the higher you build the base walls, the
better headroom you'll have. I put mine in at ground level, and being
6 foot tall, can barely stand up-right even down the middle.
Auto-vents also available from B&Q, again for about £15, and theyre
easy to fit. You could consider growing chilis, though I wouldn't
plant them right now. Stick them in a seed tray in a warm place about
february/march time, and you should be ok.

Dave Chalton