Welcome to Blade Forums, and welcome to knife making. Sounds like you're trying to start out with a relatively low budget, so I'll try to answer some of your questions accordingly.
As has already been stated, you almost certainly don't have a harden-able steel. There are a few pieces on a car or a truck that can be heat treated into a usable knife, but the frame isn't typically one of them.
That said, unless you know exactly what kind of steel you have, found steel is often a false economy. A basic carbon steel like 1084 is only around $1 per inch (sometimes less). Check out Alpha Knife Supply. They'll have what you need, including a nice assortment of handle materials. This way, you can start off with an annealed, straight and flat piece of steel that you can know an exact heat treat recipe for.
Recycled/found steel may seem cheaper, but even if it's free, you'll often spend more time (and money) dulling and breaking drill bits and saw blades, and cut off wheels, trying to beat it into submission, not to mention only being able to guess at a proper heat treat. Even if you get a decent knife out of it, it would have likely been cheaper to just buy a $10 piece of 1084.
You mentioned not changing colors while grinding/shaping. Just an FYI, but before heat treat, this really doesn't matter. You only want to watch your colors AFTER heat treat and temper.
For a basic heat treating set up, you can google "coffee can forge" for some ideas. You can do something similar with a small handful of soft firebricks. Maybe look up "simple firebrick forge". There are also some relatively cheap forges on Amazon and such, but I can't speak for how good they are. Most will likely need some modification.
Fill out your profile and let us know where you're located. There may be a local knife maker willing to help with your first heat treat or two. It may also be cheaper (at least initially) to send out for heat treat, though I can understand wanting to do it yourself.
