Go for it.I would not buy anything real expensive,though.I'm thinking you are hungry to learn and gain some experience,and you have the confidence to try.
So it is the destination vs journey thing.There is value in learning to put a good .700 or so dia hole through 6 in of steel.If you are going to use a mandrel to drive a keyway broach through,you will get to mill the mandrel.
Now,truly not to be negative,but I have enough respect to tell you where some quicksand is:With a broach such as a keyway broach,typically you would be broaching through a pulley or gear or something.Say an inch,maybe two in thickness.That will determine how much material is in the chip,and how many cutter teeth are engaged in your workpiece.Behind each cutting edge is a gullet.That is all the chip clearance you have.The chip is captive through the cut.If you are broaching a length of 5 or 6 inches,an issue is going to be the gullet will not have the room to accomodate the chip.You cannot back up and clear the chips.This same lesson about chips filling up the cutter will be important later when you are chambering.It does not work out with a chamber reamer,either,if you do not clear the chips along the way.
Another concern with a long cut using a keyway broach,with a shorter cut,only so many teeth are engaged,3 or 4,maybe 5.Each adds its own cutter pressure you will overcome with the press.With a 6 in cut,you may be pulling 10 chips.There is a limit to how much force your broach will take.
I think the length of cut and broaching limitations helped inspire the multi-lug fat bolt design,and later the receiver extension design of the Stoner AR.
I suggest you study the Weatherby fat bolt,and look close at the AR design.Study the Savage 110 and the Rem 700,See how a bolt can be fabricated by pinning a bolt head on a piece of tubing.Look at the Tubbs rifle.Then there are a number of benchrest actions that may give you some good ideas.
Some more good practice,if you would like to gain experience,find a Yugo M-48 Mauser action surplus,Then thread and chamber a .308 barrel for it.Put it together,go to the range and I suspect you will find it does not feed real well.You will gain some knowledge and experience and understanding of mag box stacking,feed rails,followers,etc.It will be a great education.,and it will be valuable as you machine and shape your own rails.
For practice,you could build a .22 or even a 32 H+R bolt gun .I think a .303 brit bbl would be about the right bore dia.
Frank De Haas has some single shot rifle action designs for the home machinist.
I hope you make chips and build something!!Don't get discouraged if you take a few detours or find a dead end.Rilke wrote something about striving to be "Defeated by greater and greater things"
Or,you could buy a Savage or Remington action and build a 308 that would shoot real well.
Keep us posted as you learn,I'm open to having you teach me something.