I beats me why this topic is so emotive. There are no pen police out there telling us what we can and cannot make our pens from or with - and no disrespect whatsoever is intended by my comments to anyone here.
Unless you have years of finely honed technique and skills, along with some seriously specialist gear, almost everyone is going to struggle to make their own nib, feed, and housing. So that's the first component that has to be bought in. Without similarly honed skills to make an eyedropper pen or a piston filler, you're going to need an ink converter too, or a piston filler mechanism, or at the very least some ink cartridges, so that's the second item. Another area you may need help with is the clip - not so hard to cut a clip from sheet metal, but the nicer clips are moulded, not pressed or cut, so perhaps a third component to buy as well.
So already, there are two, possibly three components that you have not made yourself. Are people going to turn their noises up and get sniffy because not absolutely everything was made from first principles? Undoubtedly some people will, yes, but does it matter? Not one bit!
Be true to yourself, that's the key. If you want to make a pen and every element of it from scratch, then go ahead. If you want to make parts of it yourself, but buy in some of the components, then go ahead. If you can't be bothered or don't have the skills to make your own section, then buy one in. If you only want to make the barrels yourself, then go ahead and buy a kit. Do what YOU want to do, not what other people say is allowed or not allowed. It's your pen making journey, not theirs, and you are entitled to do it however you please, without being made to feel inadequate or a cheat.
To put it in perspective, who can name any of the commercially available pen makes that actually make their own nibs. That's rhetorical by the way, but the answer is extremely few. We ourselves supply clips to at least 5 fairly well known small to medium scale commercial pen manufacturers around the globe, and quite a few less well known ones. If those things are good enough for commercial names, then, well, you know where this thought is going.