Jajone, it was pretty easy. I ended up getting some pinking shears (cheap JoAnn ones with a 40% coupon) after I was done and going back over the seams with it. Also try and get thin interfacing. The stuff I got was kind of thick but, I was in a hurry when I got it and wasn't thinking. Next time I will get the thinest I can find. Otherwise it is easy to sew. I am a beginner too. I did not do the snap crotch but, you may want to at his age.
My Ds was wearing 18 mos at 1 year too. I would say if your Ds is usually in the larger percentile height wise you will be fine. If it ends up being a little big you could always take it in a little. My Ds is 21 mos now and the size 4 fits him perfectly he is always in the 98-99% on height.
I think I would make a mess of the snap crotch. I thought about skipping the buttons on the top as well and just putting velcro and a button for looks. It looks easy to get on and off without the snaps.