A few points...
1. No, there is NOT a single version of an iPhone. This is because of the different versions of 4G that have been deployed. Details here: http://www.everymac.com/systems/apple/iphone/iphone-faq/differences-between-iphone-6-models.html That's not to say the next iPhone (the 6S presumably) won't be universal, but there's never been a universal one to date.
2. No one knows exactly when the next iPhone will be out. That said it is highly likely it will be out in mid-late September as that is the cycle that Apple seem to be in. iPhone 6 and 6 Plus hit the shops on September 19th last year.
3. In the US, iPhones have never been sold SIM-free at launch before meaning, if you bought one at launch, you had to subscribe and pay for a 2-year plan. The iPhone 6 was only available SIM free in January this year: http://www.engadget.com/2015/01/06/apple-sim-free-iphone6/ - nearly 4 months after launch.
Other than the above - good luck.