first of all abt spare parts, i dont know abt GS series if they are available in market,but im quite sure they are available at suzuki outlets (obviously the place from where u get ur bike)...
2ndly GS 150 is 150cc and HONDA cg125 is 125cc...so as a matter of common sense there is no comparison...moreover GS is 5 speed which makes it more powerful (acceleration wise as more gears=less ratios= better acceleration)
3rdly i have heard from quite a number of persons now that GS 150 goes over 120 mark easily...dont know the top speed but surely goes 125kmph.
abt the gear ratios, check the following link and read reviews by person who owned this bike...(saifi)
i think i already answered ur 5th question
ur very first 1000kms should be very very patient and controlled...it will be very healthful for ur bike's future..after 1000kms, tune it and ride it wild