Build a code to re-format large sets of excel data

I have a large sets of data in csv and excel file format. some of them are very large like 800mb. The structure of the content of the data is the same for each file with a very little minor detail that should not cause any issue. I need someone to build a code or something to merge all these raw data into 1 or 5 files (to reduce the file size). When merging, the code should generate a pre-determined headers like date, source, value etc. (i have a template for it). I've got two excel example files to show you the details which i will provide you prior to selection. Note: There won't be any milestone or full payments before the job is done. I will honour the payment once the job is done, you can check my reviews.