Welcome to the cult, amigo. Many people here will tell you that an IM in your first season is suicide. Others will proudly declare that their very first triathlon was an IM. A lot of your decision will depend on your personality as well as what you want to get out of it.
Personally, I believe in a slower build-up and would suggest that the optimal schedule for an IM is in year three or four at the earliest - that will give you a year of sprint distance racing perhaps ending with an Olympic, a year of sprint/Oly mixed racing, possibly peaking with a 1/2, and another year of one to three 1/2’s as well as the usual sprint/Oly races before you step up to the distance of a full IM. Believe me, there are a tonne of things to learn and to work on. As you go longer, nutrition and pacing become critical factors. Your body needs time to adjust to an 8-17hr day of exercise, and to the huge amount of training involved in racing an IM.
Being a runner, you have somewhat of an advantage - especially if you have a typical runner’s build. This is because it is the run that generally does the most damage given that it is the hardest on the body as well as that it starts when you’re already tired. Being a strong runner already will allow you to focus on swimming and cycling, while leaving your run in “maintenance mode”. It also means that you probably have a fairly good cardiovascular system. All of this makes your wish to do an IM in 2005 much more feasible.
If you are truly committed to completing an IM in 2005, your first year of racing, then a coach may prove to be invaluable. A good coach will steer you through many of the pitfalls us non-coached athletes make, and will allow you to make the most of your training time. (S)he will keep tabs on your progress and may be able to prevent overtraining as well as certain injuries. Search around and interview several coaches on their style, general philosophy, and personality. Find one that you like, respect, and can work with. The coach(es) of the best triathletes in the world might suck for you or me if we can’t communicate effectively. Rich Strauss has an excellent article on his website on choosing a coach: http://www.cruciblefitness.com
Having said all that, a coach is by no means necessary, even if you want to do an IM in 2005. It will be more difficult for you, but there are plenty of free plans floating around on the 'net to be had. “13 weeks to a 13hr IM” is a classic example.
If you have a tri club in your area, join it. Coach or no, you will learn a tonne from your fellow triathletes and will also have an additional source of motivation to get out and train on those days when you’d rather be in bed.
My overall recommendation is to wait a few years before jumping in to IM racing, and to just get out there and participate for the first year (uncoached) - but don’t let the ramblings of some strange dude on the internet stop you. Only you know how badly you want it, and whether or not you are willing to do what it takes. Assess yourself critically, and then make your decision based on a realistic judgement.