You want a simple graphic interface for pricing products?
As Crash points out, if you are rewriting it, you can do it in just about any language you choose. This is a simple project and easily in the scope of any of the major programming languages.
Since you have little (no?) programming experience, I would choose the easiest language to learn that can handle this kind of work. In this respect, I agree with Crash again and suggest C# using the Visual Studio IDE. It is as simple a high-level programming language as is currently available and is powerful enough to easily create what you want.
Now comes the hard part.
If you want to create a program, you'll need to design it before you start writing code. You have your uncle's program, and if it does all that you need it to do, you can simply copy his design and turn it from DOS into C#. If you know programming, that should be simple enough. If you don't, it could be quite difficult. All of the disk access and hardware access routines have changed, so you'll still have to rewrite the majority of the program.
If you aren't a programmer, and you can work on this full-time, I estimate you'll need 3-12 months (!!) to get this up and running. Your code needs to be bullet-proof because it is for a business. Expect AT LEAST 3 months and that is if you have lots of time to work on it and have programming experience.
The alternative is to find a free alternative that already does what you want. The opensource community has applications for almost every conceivable use. The time that it takes to write that program could be saved by using a program that exists already.
So .. what do you want this program to do, exactly. Be detailed. Be specific. Think in terms of what you would ask a programmer to write for you and we'll see if we can't find something out there that is capable of doing what you want.