DD1 is 3.5 and tall and skinny (30lbs soaking wet). My biggest problem is that she's mostly torso so she outgrows things like one piece bathing suits, suits and car seats a lot faster than I had expected. Right now she's ff-ing in a Scenera, but her shoulders are even with the top slot. Once they move past that she needs a new seat correct? I have no idea what to get her next that's going to last at least 18 months and is under $150. Any ideas?
Nautilus or Argos is your best bet. The top slots are 3 1/2 inches higher, if I'm remembering correctly, and it converts to a booster so it's the last seat you'll have to buy.
If you need to go cheaper, you can get a maestro (similar harness height but around $80) and plan on buying a booster down the road.
Here is a source for car seat measurements:
Since she is already forward facing, there is no reason to invest in another convertable seat. According to that site, the height of the top slots of a Scenera are only 14.75". You have a lot of options available: