depends on the height of your kid.
I have a 2 year old and a 6 month old and both can both fit the rear of a 997 quite easily and happily.
2 year old is using a Recaro - will check the exact model as not all fit, but its something like the "Young Sport" or "Young Expert".
Will confirm in due course.
My younger goes in a rear facing maxi cosi cabriofix.
I have done a bit of DIY as I didn't like the leather seats getting marked.
The bottom part of the rear is simply held down with velcro and easily lifts off.
The back part of the rear seat can be removed. I found a PDF with pictures and instructions online somewhere and I find doing this adds a couple of inches more leg room for my rear passengers.
If you only have one rear seat used, you can actually fit the bugaboo cameleon frame in the unused rear seat and the seat frame on the rear window ledge. The wheels can then go in the front boot.
Otherwise, the quinny zapp fits in the front boot by itself and you can of course plonk the cabriofix onto that.
I wouldn't buy a second hand car seat myself as you won't know the history of it - has it already been involved in an accident, etc...
Good Luck.