Newborn babies should lie flat in a pram as this protects their backs, assists with breathing and is considered much healthier for their overall development than being scrunched up at any kind of angle. This tends to mean that a chassis with a carrycot (a bit like the pram version of a Moses basket) is where babies start, though there are a range of seat reclining pushchairs now that enable 180-degree-lie-flat seat positions. As a general rule, a maximum recline-angle of anything less than this is not suitable until the baby is between three and six months old.
Many brands now offer pram and pushchair combinations where you will get both a carrycot and a seat unit plus the necessary adapters, which can then be interchanged on the same chassis. Though you will almost certainly pay more for a combination package or a “travel system” (the latter includes the baby’s first car seat plus adapters so you can slot the seat straight onto the frame), it’s worth bearing in mind that combination packages will last you from birth until around the age of three and possibly even beyond, depending on your child’s height and weight. A cheaper option at the outset may just mean you need to buy the next one up in six months’ time.