Unless you pretty much know what you want, there is a risk that even building with "all the right parts" that what you build will not be ideal for you.

I enjoy the challenge of building, and don't want to discourage you too heavily, but you need to keep one point in mind... You can't test ride a bike that hasn't been assembled.

What I did for under your price point was to buy a 2011 Trek 7.3 fx (used obviously). I then bought new wheels to be sure to hold my excessive weight, some higher volume tires that looked good (which worked out great) and some other minor parts to tweak it a little bit for my needs.Had I started from scratch, it would have taken longer, and it would have cost about the same.