You MUST have a high seatback or an adjustable head rest in EVERY car he will ride in if you get the Apex. It doesn't have enough structural support on it's own to protect the childs head and neck in a crash.
The
Graco Nautilus will last you longer in the harness as the top slot is higher than the apex. It also goes to 65 pounds. It's at the top of your price range at $150.
I think in the long run...a
Regent may be a better fit for your child and you 'll get more use for your $$. While it costs more, you won't be looking for a new harnessed seat in a yr or less when you child outgrows whatever seat they are in now.
Although if you can only afford $150 now...and can save for a
Regent, or possibly get your childs doc to write a script for a special needs child restraint---do what you need to do now to keep your child safe.
Susan