You should only use plain text files for data that doesn't need constant accessing, i.e. certain server settings, or data that is only loaded into memory on server startup and doesn't change while the server is running. Having data like guilds, which needs constant updating is really going to slow you down. While it might seem quick enough now, as the data in those files builds up, it'll take increaingly longer to find or edit something.
Plain text files are a major slowdown when modifying, because you can't just insert your few bytes of data where they need to be, instead, you need to copy the whole content of the file just to insert a small amount of data. Imagine now, you have a few hundred/thousand entries in that file - it's going to take an awful long time to modify. Your server will become unplayable in no time.
Reading from ini files isn't exactly quick either - you need to do a full on search every time. While you might be able to come up with a decent search algorithm, I doubt it'll match the performance an stability of MySQL or other DB. You're just going to be giving yourself an awful lot of work for no gain.
Now, the "easy editing" idea of ini files you have is a blatant misjudgement. Firstly, how do you find entries to edit? Sure you can find a guild by name, but what about finding guilds that have a certain number of members, or a negative fund etc. It's not going to be very simple.
Once you have found your data, what if you accidentally make a typo, for example, sticking an special character where it shouldn't be. Your code will cry, your server will crash. There's no validation checking on such things, unless you code it yourself - not worth the effort.
Now, you have your ini file, but you decide one day that, you need to add an extra field to guilds (ie, a patch adds something). Now you're fucked, because you're gonna have to do a rewrite of code, write a script that will convert the old format to the new one and copy all the data accross. It's an unmaintainable format.
Now, I'm not saying "use MySQL", although I'd reccomend it, but ini files are not the solution. Have you considered using PostgreSQL for example? It's much more stable than MySQL when handling multiple connections, timeouts etc, but has significantly slower access time (still quick enough for a game server).
Another solution might be to use SQLLite, which doesn't require a server, it just uses a plain file for the database.
Or, if you really do believe you should do everything yourself, a text file is definitely not the solution. What you need, is a random access/binary file, which, rather than inserting data at an arbitrary location, you can append it at the end, or insert into empty space, and you have a "table" saying where everything is. This is essentially what a database is doing, only a DB is much more verbose.
Not sure how you convinced yourself that using ini's would be a good idea, or if someone else convinced you, I think they were probably taking the piss. Stop while your ahead - MySQL is stable enough, you just need to fix what's wrong with it. (Official servers run on MySQL, they don't freeze up)