OP, I don't know why you want an f/6.8 or why you think you're going to be attaching a camera but I think you need to be doing a lot more reading before you even consider building a telescope, much less making an 18" or grinding a mirror. An 18" scope by itself is a moderately challenging project if you've bought the mirror but to make the mirror you really need a couple smaller mirrors under your belt and should probably have built at least one large truss tube instrument.
I am rebuilding my 24" right now. I did not grind the mirror and it is much shorter in focal length than what you are proposing. I have a shop with thousands of dollars worth of tools that I spent years acquiring and learning how to use. It is still not a project without its hiccups. I built my first large telescope, a 16" f/5, about 4 years ago - and despite what I insisted at the time it was on the verge of falling apart and worked just well enough to give me some good views before it essentially ceased to function after being dismantled for Stellafane. My subsequent 20.5" wasn't much better, as were the smaller truss tube scopes I built. It is only in the last year or two that I have been able to build a solid truss tube telescope with adequate support for the mirror, trusses that are easy and consistent in assembly and a structure that is stiff and holds collimation. And this is ignoring the mirror part! To date the largest mirror I've made successfully is a 6", and I made a 16" that had a poor figure but at least did work.
I am also helping three friends make mirrors and working on one myself - a 6" f/9. My friend Rick's 6" f/4 is the only one of these that has had absolutely nothing go wrong on the first try and he still ended up getting a scratch during fine grinding. My friend with a 10" had all sorts of problems with getting a smooth sphere, then the edge refused to polish out. And don't even get me started on the 14". The 6" f/9 is going well but I haven't had time to work on it because I'm helping with those other mirrors. And while I am not as much of an expert in making things or observing I have had years of experience and the people I am attempting to teach at least have some guidance hands-on.
If you want a big scope for not a lot of money, read Kriege & Berry and go buy yourself a used 16-18" mirror (and not one above f/5 if you can help it!). If you follow the book to a tee, you will get a very good if perhaps overweight and slightly bulky truss tube telescope. If you read some of the stuff here on CN while getting feedback and wisdom from many of the well esteemed ATMs on here you can build something a bit lighter, cheaper, more portable, etc. but you have to learn the basics first and always keep in mind basic engineering and optical concepts when attempting to try anything, and I mean anything new.
If you are dead set on grinding an 18" I would optimistically say it will take you a year or two to make at least one smaller mirror, build a telescope around it, and develop the necessary knowledge, experience and tools to make an 18" - at which point I would probably go shopping for a blank on the used market, find a porthole or get a slumped meniscus blank from somewhere. But don't even think about that yet.
Astrophotography with a Dob is not really a good idea for anything other than shooting the Moon and planets. It worries me greatly that your plans consist of obtaining a glass tabletop and getting "a digital camera feed from the eye piece area to a computer" with the completed telescope. Those do not exactly inspire confidence.