When they are older you can leave them alone in the back but as a baby it is best to sit with them.
Our baby seat is rear facing, the next size up is forward facing.
Yep, the basinette is really only for when they are newborns and cant sit. It is more comfortable for them. We changed to the seat when he was about 3.5 months old and could sit up properly. With the basinette, it is a tight squeeze to put it in the boot, but with the seat, it is fine. Definetly make sure the pram you buy fits, alot of 3 wheeler prams for instance dont fold as compact and might not fit.