Best practices for excel client database and mail merge?

Hi all. I'm a young(ish) lawyer who just started my own law firm. Unfortunately, I can't afford giant document management and billing software systems just yet. But I have enough Excel and Word know-how that I think I can build some of this stuff myself. What I lack is big-picture knowledge. Let me explain.

What I want is to automate the document creation process so I can save myself time and my clients money. I know how to set up a mail merge, and I'd like it to create all the different types of documents that I'll need for my practice. These include contracts; invoices; letters to courts, clients, and opposing lawyers; documents to file with the courts; and so on. Right now, I'm looking for a system that will make it easy to implement for each new case I get.

Here's the problem. As I thought about all the different types of documents I could create using mail merge, the number of variables began increasing. A letter to a client has variables for the address, the file number, the name of the case, etc. A document being filed with the court has different variables--docket number, court name, case style (e.g. Roe v. Wade). And a letter to the opposing counsel has yet more variables--counsel's name, law firm name, address, etc.

In doing some research today, I discovered that mail merge can't pull information from multiple workbooks (or even multiple sheets within the same workbook). So I'm left with only two options (I think). I can put every variable into one master worksheet (i.e. 1000 columns across). OR I can create multiple workbooks or worksheets (one for each client or matter) and feed that information to a master worksheet.

I don't particularly like either option. The first option has me opening a giant spreadsheet each time I get a new client and inputting data across 100 columns. The second option has me creating a template workbook and copy/pasting it and filling it out individually for each new client (and possibly still opening the giant spreadsheet each time?).

And it gets worse. Not only are there a ton of variables, but the number of variables is itself a variable. For example, some cases have multiple plaintiffs while others have just one. And some cases have multiple attorneys while others have just one. So say I create a master spreadsheet with like 15 columns dedicated to the opposing lawyer's info. This might work fine for a couple of cases, but eventually I'll get a case where there's two lawyers on the other side. Now I have a whole new set of variables that I didn't account for, and now my master list has 15 new columns.

So really the reason for this post is to see if you guys have any suggestions on how to build this system. I'm hoping that some of you have dealt with this in the past and have come up with solutions. There's even a pretty good chance that there's a simple solution that I'm just to inexperienced to see or understand. Unfortunately, it's hard to even search for solutions to these types of big-picture problems because the search terms are necessarily generic and I get a ton of false positives (or perhaps I just don't know what search terms I should be using).

By the way, I'm running Office 2013 and Windows 8.1.

Thank you in advance for all your help.