Isofix is fixed to the chassis of the vehicle so can't be installed after manufacture. And I would be hesistant to get someone who thinks they can!
Wriggles is correct, seatbelts and Isofix are the same level of safety but I would highly reccomend Isofix if you are taking a carseat in and out. The advantage of Isofix is that you can't install the carseat incorrectly like you can with seatbelt installation.
We have a capsule with Isofix and it was 100 times better than any of the other capsules friends were using with seatbelts. I could click it in and out in 1 second and always knew it was 100% clicked in, tight and safe. Highly reccomend it.
European cars and late model Japanese cars have Isofix. But you will find that Holden and Ford don't (unless its European Ford eg Focus/Fiesta) I would only buy an Isofix car from now onways. Isofix isn't just limited to expensive late model cars either. We just have a 2003 non-turbo Subaru Legacy.
Just my thoughts

We love Isofix and can't rave about it enough!