Hi and welcome!
The Nautilus is a good seat. We realy like ours. The only issues with them I can think of is that they need to be 100% on the vehicle seat, so in some vehicles like smaller trucks with a narrow back seat they can't install safely, and that they do cause some headslump for some kids. I would worry that for a younger child this might be an issue, but it does depend on the child and the way the seat installs in the vehicle. My 5.5yo has bad headslump in the Civic in this seat. He doesn't ravel longenough to fall asleep in grandma's car. I think there were also some mentions of crotch strap issues for some kids but someone else will have to elaborate on that. My kids are beanpoles so no issues here. The top slots are ~18.25" and 65lbs harness limit, then converts to a nice HBB and then a NBB. The back section for HBB/harness expires at 6yrs and the NBB at 9yrs.
A couple of other seats I would look at would be:
- Britax FrontierXT - a bit more money but eliminates most of those issues. It has 20" top slots and 65lb limit, then makes a nice HBB (adjusts very tall) with a 9yr expiry on the whole deal.
- Evenflo Maestro - a nice option if budget is an issue. Has 18" top slots but only 47lbs harness limit. The (pretty decent) booster mode does not adjust any taller so this seat will need to be followed up with another dedicated booster, probably a NBB is all you'd need. The expiry is 6 years. The biggest plus for this seat is that it is only $129, so for those who can't swing the other 2, it's a nice choice.
What vehicle are you installing in? If there is a lack of headrests and you think you will be keeping it longterm then the Frontier provides head support the longest. If you have to use a lap belt only position because other kids/adults need the shoulder belts, then again the Frontier is the best choice. Otherwise, the Nauti is just as good an option for most people, IMO.