Hornady, RCBS, Lyman all are a great choice. Stick with the O frame style very relyble and strong.
Witch ever one you choose get a second manual in my opinion the lyman manual is the best.
Now IMO if you want the Cadalac get
Forster coax press,
both a Speer and Lyman manual
your choice of dies
Lyman universal case trimmer
a powder throw (Lyman, RCBS, Horady and Lee are all good choices)
a scale I use the Lee beam scale and a digital
a powder trickler
a loading block
primer pocket cleaner
this will get you going
* The .300 mag is a good sized case. Right about an "O frame" or the Forster. I prefer the Forster
* I use a Lyman #55 powder measurer a lot, and it's a good one (find a U.S. made one). None are good for the big stick type powders he'll often want to use, so you simply trickle the loads to finish up.
* Now I'm one of many that thinks there's no better bullet seater than a Forster. What you size with his kind of an iffy decision. None work all that well. I recommend spending the extra dollars and buy an Innovative Tech sizer to go along with the other .300 die.
* I like the Hodgon manual, and the Accurate manual because they are not afraid to publish chsmber pressure specs.
* I would replace the Lyman trimmer with a Wilson. Just about any trimmer will be better than a Lyman (except for a file trim die)
* The Forster press has an excellent priming device. Better than the others for sure. But I'd go with something like a K&M hand priming tool. The Sinclair is alsoan excellent tool, but the K&M is cheaper.
* I don't even own a beam scale anymore. Have not had one for about twelve years, and really don't miss them. My first electronic scale was an RCBS that was built by Pact. Was stolen, and I simply replaced it with a Pact. Been using it for almost 15 years now. I do have a second one similar to it, and have been known to set them up in tandum. I also own a Pact BBK that I use at the range. The second scale has the inferred port for use with their auto measurer. I have it, and rarely use it. But it works very well.
I would add a good quality 1" micrometer, and a 6" digital caliper. Buy good stuff as you'll soon see the difference. The Hornaday Lock & Load case gauge is almost a must have. For checking sized cases and loaded rounds for runout, I recommend the NECO gauge.