Databases are for data. With that little volume, there will be no problem storing it all in the database and worry about what you're going to do with it later. Yes, including historical data. And for intermediate manipulations, the database already can do a lot of what you want - you just have to write the query.

Should you change your mind, it is easy enough to export it all into any format you want. And if you're worried about performance, don't. Databases are a lot faster than searching through and processing XML files. (In fact processing XML is a very silly thing to do if you care about performance.)

However one note. You'll want to think through what information you're capturing, and how to organize it. Databases really start to shine when you structure your data appropriately for them. In fact if you can, I'd suggest finding someone local who knows databases, discuss the problem with them, and have them suggest a table structure for you.