There are two kinds of rotating car seats, the kind that go from birth - 4 years (approx, based on average sized 4yo) and stay in the car all the time, these don't go onto a pram as they are too heavy and bulky.
Or you get infant carry type seats that click into a rotating base. These can tend to be a bit bigger than other kinds of infant car seat so not always the best choice for a small car. They last for approx 12-18 months and can click into the pram. These are popular currently but they are a lot more expensive than a standard infant car seat which can cost around £100-180, with optional base around £100. (the rotating ones typically cost around £250 with an extra £200 for the base).
The rotation for the infant seat is IMO a little bit of a gimmick - generally, with an infant carry type seat, you would get your baby ready and into the seat inside your house. Rotation is therefore not necessary as you can place the car seat on the floor and rotate it in any direction you like. It does come in useful either later on when the baby becomes too heavy and you leave the seat in the car permanently, or for the next stage up, when you have a toddler. The rotating bases for the infant carrier are all also compatible with a toddler rotating seat, but those rotating seats cost in the region of £300, and if you look at rotating seats which come with the base attached (don't need a separate base) they cost anywhere between £150-400, with most popular models in the £200-280 price range. I know this is getting ahead, but IME the sales people tend to bank on the fact you're not thinking ahead and just think oh great, this base will still be useful for the next seat up.
As the previous poster says, a car seat on a pram chassis won't completely replace your pram or pushchair seat, because a car seat holds the baby in quite a restricted position and is only really recommended for car travel. It sounds like perhaps you want to skip the carrycot stage? You can do this more effectively by looking at prams where the seat unit for toddlers can lie flat and parent face so that it does the job of a carrycot. I had this for all my children and really liked it - never found the carrycot part necessary, although I did also have a carrycot for DS2 and DS3. We couldn't have fitted one into the car for DS2, so we only used it for walks from home.