If you want a tourist visa that lasts for six months, you're probably talking a B1 or B1/B2 visa. But those aren't for six month visits, so much as they are for frequent or extended business-related travel.
You can't apply for a K visa while inside the U.S.
If you enter the U.S. on a tourist visa and then marry, and then seek permanent residency, you risk having the USCIS determine that you committed fraud to enter the U.S. - by entering as a tourist you declare that you have no intention of trying to become a permanent resident during your stay in the U.S.
The U.S. keeps records of each time you enter the U.S. and each time you leave. If you fail to inform U.S. immigration upon your departure (e.g., by turning in your I-94) you may have to prove your timely departure when you next seek to enter.

