Runway1, reading your post is me a year ago! Mech engineer, family, etc (last kid out of the house & established on their own ) but everything that has been posted is right on. I am four months into my first build, having grown up working on cars, bikes, Uncle Sam's airplanes, for a looong time. As I looked at cars for sale I kept saying I could build it cheaper than I could buy, the only reason the price is so high is they paid someone else to do all the work and are trying to recoup their investment, it's not the way I would have done it, etc, etc, and my wife supported my decision to launch into a build after listening to my reasons not to buy a couple of cars. I followed the Street Rod Builder "Low Buckaroo" '33 build, and convinced myself that I could do that car for $30k or less, running gel-coat for a while, spartan (no) interior, old mustang engine, tranny & gears, etc. Then you start looking at options, chasing reliability, and a hundred other excuses to change direction and before you know it you've bought a new TREMEC (stronger, more reliable, no unknown history.....), a fresh engine built specific for your car (more reliable....), new differential gears vs junk yard (is it really good???), and that sexy new EFI as opposed to old school carb and you have $13k or more just in the power train as opposed to the $1500 donor package that was put into the decision pot to rationalize the build vs buy decision.
Yes, I'm going to be able to point at things and say "I built that part from scratch", but I would have saved a lot of money buying a car instead of launching a build, especially in this economy. You can find a lot of really nice cars out there for much less than they cost to build if you shop around and narrow your search to a specific type & size. One of the hardest parts is making a detailed plan of a build and having the discipline to stick to that no matter what, other than things that don't work.