I think everyone here is just trying to make sure you really know what you let yourself in for.
I'm absolutely ready to learn and maintain. Obviously I would run countless tests and demos before going live with the system. I would document everything I do not intend on leaving as my family has had ties to this company for 15 years. However I would try to document everything i could and be as transparent with the application development/sql as possible so anybody qualified to do the work could follow afterwards.
As you say yourself, you're not just replacing the backend database. You also need to develop a new frontend.
You're basically doing software development, which is a large job and usually carried out by a team of people.
However, we don't know the scale of your project, so it might be absolutely tiny and a great opportunity for you to learn new skills on the job.
On the other hand, from what you've posted so far, it does sound like you're planning to develop a new system that is quite complex and very critical to the business. So it's a huge responsibility and a huge project - probably bigger than you'd like to admit at the moment.
I'd put together a scope analysis first of all. Ideally you also want to estimate how long it'll take, but as you don't know about GUI development, you won't be able to estimate - at least not reliably.
So, just step back and have good think about it all. It's possible to do in-house development, but it's not simple. :)