Do your research into threading etc. Know what it is you're trying to accomplish. Shop smart, don't discount used stuff.
When I built my first bike I found the bike on CL stripped everything off, bought a lightly used 10s 6600 Ultegra group on Ebay, new wheels and a few other things. I had $1200 into it including the original bike. I couldn't touch a full Ultegra bike for that price and still can't. Yeah I had to buy a few more tools, but that's an investment. I also built a bike for my daughter by picking up a donor bike to taker the parts off of and then sold the donor frame for a few bucks.
1987