A 12" scope is almost twice the size of a 8" scope when you do get it built. Until you get to the 16" size, the only problem is is you have a subcompact car for transporting the telescope. The prices, of course, for a mirror kit do get higher as you go bigger in size and you might as well figure that you are going to spend that kind of money and time compared to the price of the mirror kit. You might want to look at the San Francisco Sidewalk Astronomers website as to the sizes of telescopes. Personally, I`d recommend that you build something as you already have a scope to view with and with the limits on that little scope, you will know of what is up there to enjoy and will really want to see them better with the larger socpe.
I might also note that the 8" scope is a project in itself as far as time is concerned. The steps on a 12" will be a bit longer but not really that much more once you get the basic hogging done - you just have more glass that needs to be removed, and if you get a precurved blank then a lot of that work is already done for you. I`ve seen a mirror get ground and polished (not well) in a day by the SF Sidewalk Astro guys and silvered for viewing that night. I have also seen mirrors that got put away in the middle of working on and started up again 30 years later. Most often it`s about 2-4 weeks of evenings to do a mirror. The secret is to say to yourself that it`s just a matter of doing the stuff as there is nothing that isn`t nicely explained somewhere on the web.