Don't worry about AG. I did it as my first beer, and everything went well. By doing a few extract batches you know a lot more about the brewing process than I did when I tried. If you build an MLT (10 gal), and I assume you've researched the process on this forum, it will go very smoothly.
Regarding the chiller, don't worry about using a 25' 3/8 IC. A 50' IC would be quicker (maybe overkill for a 5g batch), but a 25' IC is better than not using one.
You'll do fine - it's fun.