I only discovered Construct 2 a couple months ago, and so far I have seen licenses go on sale twice. So, you probably wont have to wait too long for the next sale...
There is no functional difference between the personal and business versions. You only need the business version when you start to make a lot of money - and then it shouldn't be a problem! :)
When licenses go on sale, the upgrade from personal to business license has also gone on sale too, so there really is no need to start out with a business license first if money it tight.