I was in the same situation for several years. Company wanted to save money, so as people move on to less stressful positions, I was "allowed" to put on more and more hats. Mail, Exchange, SQL, Migration/AD implementation, Backup, Firewall, mail/FAX solution, server, WAN, remote support, OWA, VPN....you know the drill. The more I took on (promises of bonus too), the more stuff was heaped on the plate.
You are in a no-win situation, if you succeed...it's obvious that it couldn't have been "that" hard...and if you fail, you'll be blamed for wasting time, effort on something beyond your ability. Since you have already taken on the task, you best recourse is to hire someone to take on the day-to-day system management. Then you can concentrate on completing the project. Carve out either the evening (or morning) to devote, exclusively to the development project, spend the other part of the day, working with your surrogate (an intern would be perfect for that), so user support, confidence is maintained.
Not to doubt your ambitions, I seriously believe this is a bit much for untrained, first time junior programmer to attempt. But since you must, at least give your self the best chance for success.
If you run into insurmountable problems, then bring in a consultant to help with that portion of the code. Clear this contingence with your manager, make it simple so he can understand that some things are better farmed out, rather than spending the time for you to get up to speed. Your job would be better as a project manager/analyst, than strictly as a programmer?unless that is your ultimate goal. I think of programming much the same as I would think of turning my system over to someone working at the help desk. Its not as easy as it looks, especially if you are doing it right, with documentation, error control and proper error checking, graceful shutdown. It could be a case where you don?t even know what you don?t know.
Bottom line; you really need to give some thought as to when it is better to delegate and what to delegate. The visibility of this project, now that you have accepted it, makes it very important that you complete it...one way or the other. Even if you have to go to you manager with a packaged solution (and an explanation) it will be better than trying to learn coding and maintaining your user satisfaction level at the expense of your long term (mental) health.