I'm in the middle of a similar project myself. I started off not wanting to waste money on an engineer. Trust my competent GC instead. But then the planning officer queried a couple of elements of the design and I got the engineer out - holy cow, good job I did. Suffice it to say I am now having to rebuild the roof and top floor walls completely. Lucky the roof didn't land on my head when we had all that snow this winter.
$17k seems cheap to me. I'm in MN, and local trades are slow, willing to price their time pretty keenly. My project's about 40% bigger than yours, and I'm looking at around $33k for a comparable package (I've deducted windows and siding). Roughly half is labor. I also intend to do all the interior finishing, insulation myself (that'll still cost me a few grand, maybe 5). If you go into your local Menards or equivalent, they'll help you put a material list together. I normally underestimate waste removal ($360 for a 20 yard demo load here, and I will need 4-5), permit fees, equipment rental (including crane to lift the roof trusses) and consumables like nails, blades, etc. Wow, $17k definitely seems low to me. He's not building any contingency in there. Is he reliable/trustworthy? Do a service magic request and get at least 3 more quotes.
Good luck, and let us know how things proceed.