I like where your intentions are at. Unfortunately, unless you already have a good amount of experience fitting parts, you will likely not save any money over a full custom. There will be parts you mess up and have to buy thrice (read safety), the headache that comes along with things not going as planned, in addition to learning things the hard way. This is assuming you haven't done this before. If you have, well, this probably wouldn't be a thread at all.
The real question is: For X amount of money, do you want a full custom build from a reputable smith, or would you prefer the experience that comes along with building your own, without as high a regard for quality in the finished product.
Same price either way.
One option offers holding someone else accountable if things aren't up to snuff, which isn't likely to begin with.
The other option quite possibly ends up a $4k pile of paper weights in the shape of pistol parts.