I would actually suggest Handcent:
Handcent SMS - Android app on AppBrain over anything else. It is a wonderful, highly customizable app for SMS and MMS messaging.
Go to
http://www.gmail.com and sign in. On the left side of the page, look for a link that says "contacts" and click on it. In the top left corner of this new segmented window, you will see two buttons. One has the image of a person with a + and the second has the image of two people and a +. The second image button is the one you want to click on. You may get a message at the top of your browser that says the webpage wants to run a script. If you get that message, click on it and choose to run the script. A small window should open up that says "Script Prompt: What would you like to name the new group?" and you will see a text box just below that message. Enter in the text box something along the lines of 'All Contacts List" (For the purpose of this tutorial, we will use "All Contacts List") and then click "ok". Next, look at your list of contacts and you will see all of that they all have a check box to the left of each contact. Go through the list and check the check box for the contacts you wish to add to this group (you may also simply click the "all" located just above the list of contacts). Finally, look to the right of the list of contacts and you will see a button called "groups". Click on it and then click on "All Contacts List" under "Add To...".
You have now added all of your contacts to a group named "All Contacts List".
Now to take care of the app details:
Make sure you turn off the notifications for the stock messaging app when you download this app. Play around with all of the customization options and get it set to just the way you want it (conversation bubbles, bubble colors, max MMS size, etc).
Once you are done customizing it, go to the front or main page of the app. If you need to, completely back out of the app then open it again. Look in the top left hand corner of the app and you will see what looks like a piece of paper with a folded corner and a pen or pencil. Touch that and it will open a new message. In the "To" field, you will see a green + with a circle around it. Touch that and it will open all of your contacts. The middle option on the top of this page is for groups. Touch that option and then select the group you created and called "All Contacts List". Then press the back button on the bottom of your droid once to return to the compose message page (the page with the "to" field and the green +).
Finally, compose your text message and then press send. It will then send the same message to all of your contacts individually. This may take some time as you said you had more than 100 contacts.
I have done this multiple times and it works wonders. You can even create multiple, different groups and just send texts to just the people in the groups you select. For example, I have groups set up for coworkers, friends, family, employees, class mates (a group for each class that i change each semester), clients, etc.
I hope this helps you accomplish what you are wishing to accomplish. If you have any questions, feel free to ask or pm me.
Kratos