Convertible car seat recommendations?

My parents need a reliable convertible car seat for my unborn son and 1yo niece. They'd preferably like to spend no more than $150, and they'd like something with a lifespan of around 10 years so they don't have to replace it every time they get a new grandchild in the future. My parents won't be leaving it in their car all the time because they go on trips often and have two dogs, so they'd also like something that is very easy to use so they can quickly buckle the seat in or take it out when necessary. My parents currently own a Subaru Outback if that makes any difference. Any suggestions are appreciated!