You've had a great idea already, in having a calligrapher hand-write the invitations. Not only is this a beautiful and elegant way to have a small quantity of invitations produced, it's relatively cost-effective. You might want to check your local area for calligraphers (the phone book, or wedding informational websites). Call them up, ask for pricing for 25 hand-written invitations, and ask to see samples. Most calligraphers are happy to send you samples of their work, so that you can feel confident when hiring them. You might also ask about papers; the calligraphers may have resources for beautiful papers, or suggest papers that they've worked with previously. Be sure to ask about the cost of a response card, if you want the calligrapher to create one of those, as well as the cost of a return address for both the response envelope and the mailing envelope, as well as the guests' address.
As far as creating something "special" on a computer, there are a couple of things I would suggest. First, head to your local craft store, book store and/or art supply store, and look for scrap booking, stamp, or invitation books. There are many of these types of books out there, and they're full of wonderful ideas and instructions for creating something unique and truly special for your wedding, even with your computer. Craft and art supply shops often have many items that may be used to embellish a simple printed invitation, such as rubber stamps and embossing powder, shaped punches, and colored paper which may be layered to create a beautiful effect.
If you're not particularly crafty and wouldn't be comfortable creating something yourself, consider approaching a designer. Many designers will create an invitation design in PDF format (an electronic file format that takes only a free program -- Acrobat Reader -- to open and print) that you may print out on your own, or at a local Kinko's-type shop. This would be less costly (in most cases -- you'll need to ask for a quote from the designer) than having the invitations professionally printed, but will still have a professional look.
Creating your own invitations can be a great way to cut down expenses. Since you are not quite sure what style of invitation you want, I would suggest doing two things: first, take an afternoon with your fiance, and spend some time at a local stationer's, just looking through the books of invitations. You will find, as you go through the books, that both of you will be able to pick out things you like about the invitations, as well as things you don't. (For example, you may find that you both like very ornate scripts, or vellum overlay, but dislike green ink for the text.)
You also may want to jump online, and join an online community where there are bulletin boards devoted to invitations exclusively. These communities foster a creative spirit, with many members providing photos of their creations, as well as instructions/templates so that others may create the same or similar pieces.