Many people have said that they just project the film onto a screen and video tape that. It is a cheap and successful way of doing it. Otherwise you probably have to send it out to a costly professional service.
I have to wonder if service providers can give you a digital file on DVD and if it would be a Windows codec only that would require you to convert it for editing on your Mac. If you go with a service, start by sending only one or two reels so that you can judge how well their service works for you.
The format is a good question. I'd suspect since you are a Mac user, the Apple Intermediate Codec would be a good choice. iMovie and the Final Cut applications can use that natively for editing. The thing is, your original would likely not be that codec. Keeping the original is always advised.
With HD becoming more popular, it might be worth it to have your new digital content done in HD. I would suspect the service providers only do standard definition since HD is relatively new to the consumer market.
Scanners are your best solution for the slides because you'll get more information and a better result because of it. Some flatbed scanners have appropriate hardware fittings, but I'd prefer a dedicated 35mm film/slide scanner.
Good luck on this project.