1. How old is your child, and what's your child's weight?
2. Why must so adamant want ISOFIX?

1. Firstly congratulations.
Secondly, not necessary must get those with ISOFIX features. ISOFIX features is something the child cradle or seat has that would easier "lock into" the car's seats. It's something "nice" to have, but not mandatory. You can still get 1 with ISOFIX features, use it in your partner's car, and still use it in your own car.
A lot of baby outlets have these car seat demo which you can ask the staff to demo to you on how you can fasten/secure your baby carrier onto the seat.

What you need at this moment is just to SECURE the baby carrier onto the seat, with the car's normal seat belts also can.
As long as the carrier is, or can be secured properly without it being able to move around on its own, then it should be OK.
Also, no person in their right mind would be driving like a maniac with a newborn baby in the car, right?