Hi - I am a IT professional with 20 years of experience of which 18 years were with Infosys, a Tier-1 IT company based out of India. Through the company i have worked for customers like Microsoft, TI, Boeing, Apple and delivered professional output. It might be worthwhile to state that i have 2 patents in US.
We term the type of project you have described as re-engineering project. There are various strategies to execute such projects which can be assessed only upon getting into the specifics of the applicaiton under question.
At the outset for the application where you have code, we can do a system appreciation through code walk through and begin the technical work. For the other one where only exe is available we need to search for availability of SMEs or users of the applications and table the requirements, discuss with stakeholders and assess the requirements, look for enhancements and them build it in such a way that it has a longer life from now.
Would be happy to discuss further and take it forward given an opportunity. The cost estimate given is highly tentative and i am taking the value freelancer gives as default.
Regards,