Hate to be a damper, but the reality is that the car alone will cost you at least 3k to build if you go bargain hunting on everything. Typically it's 5k+. Plus your own personal safety gear. I don't say this to sway you from the project, it's just good to know what you're getting into.
If you're determined to do this all yourself more power to you, it will be a learning process for sure. Most of it is easy. Go read the rules, then read them again, and then one more time just to make sure you don't miss something. They aren't long. Take some time, read everything you can on the car you pick. Read everything you can find about common failures, fixes, etc. Replace brakes, check the fuel system, check all the electrical, check the frame for rot.
The cage is going to be your biggest challenge, no matter what route you take. There are three options. First, just pay someone to build it. Expect to pay $1200-$2500 depending on the shop and how well the cage fits the car. You can get a cheaper one that will pass tech, but it might not fit super tight to the car. Or you can pay more for a completely overbuilt cage with lots of extra bars that just make it safer.
If you're trying to do it yourself you're in for some fun. I'm about 90% done building my first cage, and it's been 3 weeks of working every night. I watched every youtube video I could find, read every forum post I could find, and then still panicked before I started about how I was going to do it. Materials are going to run you $500-$700 depending on how good of a deal you find, and how many bars you mess up. I bought 4 sticks of 1.75x.12 tubing totaling about 94ft, and I used about 80ft of it. I screwed up one forward bar, but was able to salvage that as a door bar and smaller piece of harness bar. I got incredibly lucky that I didn't mess up any other bars. I have a number of extra not required bars in my cage because it makes me feel safer.
The third option, there are a number of places that will sell you a pre-bent cage kit based on measurements you send them. And you can usually get the kit shipped to your door for $800-$900. It will be bare bones, but legal. You'll have to do notching and final fitment, but at least all the bends are done. This is probably the best route for savings provided you have someone that can weld. Examples of places are rollcagecomponents.com and S&W race cars.
A word on those last two options. You need someone that can weld. I would not try to weld my own cage if I had only learned a week previously. This is equipment that could save your life. It took a lot of self convincing to get myself comfortable with the idea of welding my own cage. I did a lot of practice on scrap tubing, a lot of weld testing, and then even more practice. And then you still have challenges because a cage gives you a lot of out of position welding. Laying on the car floor with your head in the footwell barely able to see what you're doing while welding one handed is an exciting challenge. Not to say you can't learn welding and do it, just again giving you an idea what you're up against.
Now, and alternate option, buy a car that's already prepped. You'll need all the money up front, and you don't get to build it yourself, but you'll have a turn key car that is ready to go (mostly).
20+ Time Loser
FutilityMotorsport Abandoned E36 Build2008 Saab 9-5Aero WagonRetired -
1989 Dodge Daytona Shelby 2011-2015 "Lifetime Award for Lack of Achievement" IOE, 3X I got screwed, Organizer's Choice