Re: The ONE thing I can not figure out!
You won't be able to get one seat that lasts from newborn through out of seats altogether. But, there are several convertible seats that will last from newborn through age 5-6, at which point you can move the child into a booster.
These options should all work (and prices top out around $200). If you're going to use the seat for 5-6 years, every day, then try it out first and make sure you like it!
*Britax roundabout 55 or marathon 70 - you *may* need the infant positioner for a newborn, those cost about $15
*Evenflo triumph 65 or momentum 65
*The First Years/Lamaze Truefit
*Maxi Cosi Pria (might be stretching your budget)