LMGT4's in black were limited and only 18"
Yes you can just stick 19's on without any issues, I did.
No issues with brake calipers if you buy wheels with the correct offsets etc.
Offset is how far they stick out of the arches. +18 is very aggressive and will be sticking outside of the car. +38 will be inside the arches and look pants, think Jag E-type.
Spacers are used as a cheaper way getting the wheels to stick out more. EG a set of wheels with an offset of +38 may require a set of spacers to get them further of of the wheel arch.
Ride shouldn't be any higher as even though the wheels are bigger, the aspect ratio of the tyres goes from 45 to 35.
HTH
Bloody hell, that must be your longest post this year!
And his most informative