Need some advice re temp car seats

[Melty]

Friend coming to stay from overseas with 2yr and 4yr old for 3 month period.
Both kids small, 2yr 11kg, 4yr 15kg but tall for age. (Won't be bringing her own seats)

Don't have a car and mostly won't be needing one as London based.
However occasionally may use Zipcar.

I want to try and sort out seats for the just in case. I have a couple of months before they arrive.

Does any one have any suggestions as to what/where I should look.
Are second hand seats a no no? (Not completely against them but am worried about knowing if they've been in an accident)

What sort of budget should I realistically be looking at?

Will have to be forward facing as thats what they have been using and I cant imagine the high jinks that would ensue if we went rear facing now.

the 2 year old will need a harnessed seat but the 4 year old could use a high backed booster secured with the adult belt. There are seats to suit mist budgets - how much are you looking to spend?

could do the 2 year old

for 4 year old

They look ok actually. Price wise I hadnt thought about it but they are very reasonable. For short journeys they would be ideal and unlikely to break the bank. Thanks. :)