The 1909 Argentine actions are very well-made and finished, but, in common with all the earlier M98 actions, are not so durable when used as the basis for more modern higher-pressure cartridges such as the .300 Win. Mag. In later (post - WW1) actions, the heattreatment was improved to give a deeper case hardening on the wear surfaces. Custom rifles built on these actions as-made often suffer from setback of the locking lugs into their seats in the receiver. There are specialists who will re-heattreat the action to prevent this, but this is an extra expense, and the results are not always acceptable (plus, there is always a risk of warpage or cracking in heattreatment - not lessened by the age and previous use of these actions).
Of the currently-available M98 military actions, the VZ24 is probably the best and most economical one available - they are all properly heattreated and well made.
However, since the cost of the completed rifle will include the costs of modifying the military action to accept and function with the .300 magnum, and incorporate the features found in commercial Mausers, why not start with one of those? New ones are available, made for the magnum family of cartridges, and used commercial Mauser-actioned rifles can be found at reasonable prices.
The truth is, unless you can accomplish most of the work yourself, modifying any military action to 'sporterize' it is, today, uneconomic, and the resulting rifle, no matter how satisfying to the owner, is not better in any respect than one built on a commercial action, and probably more expensive.