Offline solution and data sync up

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The requirement is given below;

Step 1 : User logs in to a system and download 5 lakhs records to his local machine (the system connects to a database located in a server - SQL Server database is the backend).
Step 2 : User disconnect from the network and he work on this local data. He will edit row by row and save it to his local machine. This process will go for weeks or even months. (currently the users are doing in excel - completely manual work - no storage - so we should get rid of excel usage)
Step 3 : User finishes editing all the 5 lakhs records and he is ready to sync it back to the remote database. He submits the data to the system again and the system has the capability to sync it back with its central or remote database.
Step 4 : When the next cycle comes user again download new set of data. Multiple users have to work on different set of records (we have an approach currently to handle security in manipulating data).

We are thinking of the below inputs for the solution;

- Winodws application is more suitable for this requirement considering the fact that in a web application also we need to deal with a local db which is challenging. Again we need some solid points to derive a conclusion (in terms of performance, mitigating the challenges in this particular requirement).
- With respect to downloading of data, we are thinking to have it in a local database and provide a solution so that the user can easily navigate record by record so that he can modify the details without connecting to the network.
- Need to look at how the same application can deal with local database.

Any suggestions on how to go with the solution?