Prams are waaaay more complicated than I ever though possible. A lot depends on your circumstances. If you drive it needs to fit in your boot. If you walk a lot you need decent suspension and good quality wheels. If you use public transport a lot your best with something compact and easily folded if necessary.
Babies under 6 months (or before they came sit up unaided) should lie completely flat as it's better for their spines.
We decided against a travel system with a big carry cot and separate seat unit because we didn't have the space to store one whilst the other was in use. However, I know some carry cots are suitable for overnight sleeping so would have dual use.
We bought a pramette, where the seat unit converts carry cot style by fastening and unfastening straps underneath. We got a good 18 months exclusive use from that, then bought an umbrella fold buggy and alternated between the 2 until he was walking full time.
I'd suggest visiting some pram shops and having a play about. Independents tend to be better as they often offer lay away plans and will store the pram until you're ready to collect it. If you have a car make sure you try it in your boot coz some aren't very compact when folded.