Is there any way that you can just let nature take it's course and let them flower outside?

If you can't and absolutely have to do inside HPS, or high pressure sodium, lights are best for flowering. You'll need a 400w- preferably -for 6 plants. You could flower with floros, or envirolites, the price and the heat will be much less but so will the yield. Use a floro in the red spectrum if you go this route. Once the plant(s) is sexually mature, displayed by alternating nodes, it is ready for flower. And you said that you have all the space your want, but keep in mind that that once put in flower a plant will double or triple in size.
A lot of good information can be found in the faq's too. Hope that helped.

Good Luck,
:jointsmile: Bree