First, just make one out of mild steel, and try it out.
If your work is nice and cherry red, and you dont need a thousand pieces, you can probably get away with it.
I have found that plain old A36, which is mild steel from the steel yard, has so much variation in it these days because it is mostly electricly remelted scrap, that it has a higher carbon content than "mild" did 20 years ago, and that it can actually be hardened to a small degree just by heating to red and water quenching. This may be hard enough.

If you want to get serious about making tooling, yes, you need a higher carbon steel. There are air hardening, water hardening, and oil hardening steels, with different techniques used for each.
A good hot working steel is 4340, which needs to be oil quenched. This is what Grant uses for all the off center forge tooling that he sells at kaynes- www.blacksmithdepot.com

A lot of blacksmiths us S-7, or some H series steels as well. Tooling is an art and a science, and it takes some time and experimentation to get it down.

Does grant already make a tool that does what you want?
He makes quite a few, in both single sided and spring swages.

Jake, I make a lot of my tooling from mild steel then either quench in water, or case harden them. As long as you are forging mild steel thats red hot these will last a long time.

Another cheap way to go is go to harbor freight and buy their cheap hammers ( this is about all I buy there) these have good steel in them and make nice textureing tools.

Or as Ries said go to Kane and Son's, they have a great selection.

"GRIZZ"

Jake,

I make a lot of my hot-work texturing tools and the like out of old truck and car axles. They're generally something on the order of 4140 steel, it seems, and they work fine for most purposes. If I need something to last a thousand cycles or it needs to be capable of working cold steel, then I use S-7 and heat treat it as best I can in my forge. For hot chisels and such that absorb a LOT of heat, I usually use H-13.

5160 spring stock, like found in truck leaf springs and many car coil springs, makes a pretty satisfactory steel for many punches, texturing dies and the like.

Case hardened mild does a pretty damn good job for me, and you don't have to worry about making the stake too brittle as you might if you didn't get the temper right on a tool steel stake.
That said, some of the best punches I've made were from the tough springy things they use to clamp railway lines to the wooden sleepers (is that a tie?) when I found a load at a scrap yard.
Will Jones, www.ironwill.org.uk

I'm with Ries, try mild steel first. You can do lot with mild steel if no shape edges are involved. Case hardening is the next step up and will raise the outside of the mild steels carbon content, with the effect of making the tools more durable. The added bonus is it inexpensive.

The next step up for me, is to use 4130/40. I use the 4140 as that's what the local steel company sells. I make almost all my hot tools out of it and even though it's a oil hardening steel, I just use water most of the time. The reason for liking 4140 is it's very forgiving. You really have to do a lot wrong to get it to fail.

Since I'm on a roll, lastly I'll use W-2 for small tools that need a fine edge.

And that's it. I don't use any of the other alloys as I try to keep life as simple as possible. That's not to say that those alloys aren't pretty awesome. A friend made some chisels out of S-7 and he was cutting mild steel like butter. The difference is, I can forge out 4140, heat it up in the forge, lay it to the side to cool, harden and the temper without any special effort. He spent all night following the annealing process for S-7, but got for his effort a far superior set of tools

As for the special kiln? No, for what we do, a coal or gas forge works fine. If you start using the higher alloys and want to get the full advantage out of them, then you will need a way of controlling the temperature of the annealing, hardening, and tempering process.

I wouldn't try welding harden tool steel to mild steel. While I've never done it, I immage the effect would be cracking of the tools steel.

So having written all that, let me get to answering the question. My advice is to use mild steel. (all of my bottom tools are mild steel) Just weld whatever piece you have for texturing to the hardy hole base and give it a try. It will probably be all you'll need to do. If not case harden it. You can get case hardening compound from any of the industrial supply companies or most welding supply stores. The brand I have is Kasenit

Thanks everyone for your advice, I think I'll try the mild steel and case harden if I need to first. I really only need it for this one project so if it'll get me through that I'll be fine. If not I'll try the tool steel or go look for springs and axles. I'll let you know how it works.

Jake