I am new to working with CMS software but have some amateur knowledge of XHTML, PHP, MySQL, etc. but I decided it was finally time to stop reinventing the wheel for each web site I work on. (This is not a profession, but a hobby.)

I need to build a web site for my family's small business, and have done a lot of work on it already. Most of the site will be static with information about the company and current projects (although I suppose I could make that dynamic too), but he wants an area where clients can log on and see pictures, information, and updates regarding their purchases (vacant lots becoming their new homes). While this could be done with static pages as well (since there are only going to be around 30 clients or so) I want to do this dynamically in case a new operation has many more clients, and also so that a secretary unfamiliar with HTML, etc. can log-on to the system and update indiviual client pages using a simple template. The clients should only be able to see their information (pictures primarily) and not the information of any other client.

I started designing something in PHP and MySQL to do this but I soon discovered it would be a long and time-consuming project. Do you think XOOPS would work here or would some other program be more appropriate? I installed XOOPS yesterday and am trying to experiment with it now. Thanks in advance for your advice.

First of all, welcome to the XOOPS community!

I have found XOOPS to be very flexible, I have never wanted to do something like this, not sure if it would be easier to create a new group for every person or if there is a module about which could allow you to set different permissions per different user rather than group.
It is worth having a search around as what I love the most is the design flexibility, you can make any design work in xoops, I personally have not a clue of php (I do plan on learning but am struggling) so I currently get xoopsdesign.com to do all of my projects, I design the layout in photoshop and they create the theme.html/style.css etc...
I would reccomend them highly if you do struggle with theming.
I understand you have worked with php before, this could be helpful as you have an idea of how to edit modules to how you like them.
Overall I definitely think that you should experiment for a few weeks with xoops, as when I first started using it, I was confused as all I got was the white page with a small blue login block But after a little messing around worked out everything and have not used any other portal/cms ever since.
I have personally tried and tested about 30 other portal/cms scripts and I still think XOOPS is top!
If you ever do struggle with anything, please do contact me and I will see if I can help. I don't know everything about xoops, but after asking a lot of questions, I am getting to know it like the back of my hand.
STICK WITH IT!
Cheers
~seolio~