Look it up in the chart:
http://www.airsoftcanada.com/SpringChart.html
And yes you should probably use a 9.6V battery if it's a mini.
A 8.4 large would probably be fine. Also a stock TM motor should be fine, just depends on the battery.
Your gun has a V3 mechbox right? You shouldn't have to buy a reinforced gearbox. Run it till it breaks, which might not be for a long time.