An off-the shelf fixie is a great starter bike. You get a good picture for what you like and dislike, and learn a frame of reference. If you love it, awesome. If you don't, you have an easily re-sellable bike. If you build up a conversion as the first, you won't get anywhere near the money out of it.

If anything, make your second bike your labor of love. I've done that, and I'm about one week away from getting it, and since I know a lot more, it's 10x the bike I would have built my first time out.