I am just fitting belts almost as we talk:-
My 86 car has mounting points for 3-point, but only static. If you want to go for inertia belts you need a mounting point fabricated on the rear wheel arch. As it happens I phoned Quickfit-SBS and got a quote for this, I think it was going to cost me £250 fitted for two inertias in the back. I decided it wasn't worth it for this car but will be sending the 911 there.
So I got some static 3 points for about £20 each and I expect these to be fine.
As to child seats, i used a Romer prince in the back of my 911 until our lad was too big, 20Kg I think. Quickfit-sbs gave some advise on the next seat which this is a maxi-cosi I think, can have a look when I get home. This seat has a removable back on it so that it can become a booster.
As others have sad you have to use a booster until aged 12 or a certian hight. I am very tall and so is my lad, so even though he is five he is the size of an 8 year old. Both Quick-fit and myself think he will fit better WITHOUT a booster, and so I have tried him in both cars and this is the case. With the booster his head is way above the seat back.
Obviously this is illegal, but I think it is safer without the booster and then worth having an explanantion with a jobs-worth copper should the need ever arise. I care more about his safey than the law. These seats in these cars are completly different to any other car.