Best car seat that clips on a frame

hi all, really hoping someone can help me! 

my amazing mum is a foster carer and has just taken in the baby sibling for the two children she’s already caring for. 

Shes looking to find a lightweight, easy to use frame and car seat. She has a pram which she has been using most of the time but for school runs she really needs a seat that she can just take out and clip on a frame. 

The two she already has are at different schools 20 mins apart so being able to do collections quickly is a must.

 the other factor to consider is cost as she will have to fund this herself. 

any advice on your experience with reasonably priced seats/frames or reputable second hand places to buy would be very gratefully received! 

thanks so much xx