Sounds like an exciting project! Ethical PR Girl has given you some great points to consider. I would like to build on the comments re project timings a little...my recommendation would be that you allow plenty of time to get the look and feel (the design) of your publication spot on...but once you have your design right, you still need to allow time for typesetting, proofreading and reprographics (before printing) - be careful not to miss this from your timeplan. I have a background in design and reprographics, and it's fair to say that this is the less sexy part of the process that sometimes get forgotten about, but it is still hugely important.
If you decide to use a separate design agency and print agency, you might want to consider at what point you send the design over to the printer. Design agencies are likely to offer to do all of the page layouts for you, but they will be pricey. Many printers will have their own artwork studios, and they too should be able to handle all of the typesetting, providing there is a style guide supplied by your creative design agency. Typically, a printer's studio rates will be lower than a design agencies studio rates.
Hope this helps.
Wishing you lots of luck with your exciting new magazine launch!