Check out his references, see if he will give you some additional references from when he was in business before. Take a look at the finished jobs and the quality of workmanship. If you are satisfied and comfortable that he will do a good job for you then you got a good deal.
Situations change and hopefully he is being truthful with you as to why he has started his business back. If Okna is as good a window and company as many state, I would hope that if you had any warranty problems in the future that they would stand behind their product and warranty.
Okna makes an excellent window and is a great company that will stand behind their warranty. The thing is, they have no control over how the window is installed. If an installer butchers an installation causing the window to leak water, air, or operate poorly, that is the responsibility of the installer. In addition, even if there is a problem with the product itself, the manufacturer is only going to provide the new part(s) under the warranty . If the original installer is no longer around, you will be stuck looking for another dealer to fix the issue, who will then likely charge $ for his services.
I don't know the details of this situation, installer, etc., but shopping on price alone can be a very dangerous proposition, even if you are getting a great product. The lowest price rarely = the best value.