Deffinately not too expensive for a first timer. As a bike shop employee for the past five years, I have seen that most people who are looking to buy a bike in order to be competitive with it are not normally the kind of people who are going to give up and let the bike collect dust after one season of use. When a customer mentions that they want to be competitive, I normally suggest buying at least a mid level bike, such as the madone 4.5, as many dedicated athletes tend to outgrow the entry level bikes within a year, and then they end up buying something nicer and are more or less out the money that they invested in the original bike.
That being said, lets talk about the bike itself. The 4.5 is a very well spec'ed bike for that price point, with its 105 10s group and full carbon frame (with the exception of the seat tube having a thin aluminum sleeve in it). The frame itself uses what trek calls their TCT carbon, which is produced overseas in order to keep the price lower. It is a very solid bike for someone looking to begin their cycling career, with one minor exception. The 4.5 is only offered in "performance" fit, which is a more relaxed geometry than the "pro" fit that is offered as an option on the 5 and 6 series bikes. The performance fit has a 3cm taller headtube, a longer wheelbase, a shorter reach, and more relaxed seat tube and head tube angles. It is built to ride a bit more "comfortably" than the pro fit bikes and will not corner as sharply or handle as nimbly as the pro fit frames. Some people do like this sort of geometry as entry level riders since it does seem to feel a bit more stable to newcomers to cycling. The geometry of the 4.5 may help you relax a bit more while you gain the technical experience that is essential to the sport, but I would suggest that you at least test ride a bike with more aggressive geometry to see if you would be comfortable with it, as it is something that you may want down the road.
Hope this helped a bit.
Edit: Forgot to touch on the other questions.
Accessories that you CAN NOT forget:
A small saddle bag
Spare tube(s)
Tire levers
A floor pump with a gauge
A pump to carry with you on the bike (preferably a CO2 pump for ease of use and space savings)
2 bottle cages
Water bottles
Helmet.
A GOOD pair of cycling shorts. Don't waste your money on cheap shorts, you can thank me later on that one.
Jerseys, preferably with 3 open pockets in the back. The zippered kind can be a hassle, especially when wearing full fingered gloves.
Lights, is you plan on riding in the dark.
Clipless pedals aren't essential, but they are HIGHLY recommended in order to increase efficiency. If and when you go clipless make sure to invest in a good pair of shoes. Shoes and shorts can make a HUGE difference in your comfort level on the bike so those are the two places that you really don't want to skimp.
Some sort of athletic sunglasses that offer good coverage, and preferably have replacable lenses for different light/weather conditions.
As far as the extended warranty goes, what does the shop offer as packages and what are the prices? Most shops in this area offer anywhere from 1 year to lifetime general tune ups for free.
The RS300 is not compatable with speed or cadance sensors.
