You can buy software development software at stores such as Best Buy, Future Shop, and others. You can ask for assistance in looking for the software you want and wish to purchase.
Because software can be very difficult and complex. We need the SDLC as a framework to guide the development to make it more systematic and efficient.
We will be able to tell how long it will take...
This is based on your opinion. This depends to what extent you are doing for accounting. Quickbooks seems like a good basic accounting software based on the link results.