Car seat advice please?

[Wallace]

My ds2 (7 months) has just grown out of his stage 0 infant car seat. I have put him in dd's old combination car seat, rear-facing, because he is not old enough to go forward facing.

Now dd's seat is about 4/5 years old so obviously not ideal. Do I buy a combination seat (very expensive!) so I can keep ds2 rear facing as long as possible, then turn the seat round. Or should I keep him in dd's old seat until he is 9 months (would like to keep him rear-facing a bit longer than that if poss) then buy a forward facing seat(would be a bit cheaper)?

Even if it's 5yrs old it should still be safe unless it's been in a crash, is broken or is showing wear on vital bits like the straps.

Ooh thank you. Should be okay then to keep him in it for as long as poss before buying a forward facing seat