We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember mo saying something
like:

1) if I get an SDS is there any real reason to keep my normal drill? They
still work like normal drills on wood/metal etc AFAIK - downside is that
they are much bigger!

Keep your old drill for those times when you might need a bit more
precision. Otoh, I use the SDS drill for most things and find the
'wobbly' chuck thing to be over-stated. Let's face it, if you really
need some drilling precision you need a pillar drill.

2) Any difference between the difference type shapes? i.e the one I linked
above as opposed to the normal shaped drills that don;t have the fat bit
infront of the handle

Cheapness of construction for the Chinese factories. The bulky style
means it's easier to use (and fit in) larger gears of cheaper material.
The really properly heavy-duty drills use this style, but that's because
they need the room for proper HD gearing. The cheap ones copied this
style because it made sense for them and also made it look as if they
were HD, where they are not.

3) Does the weight 5kg or 6kg just mean the higher weight the more heavy
duty?

No. As I said above - the reverse is true for consumer items. A proper
HD drill/breaker is heavy and bulky (SDS Max), but decent makes of
consumer SDS are normally half the weight of the cheapies.