Going back to the question in the op. The answer is that depends.
First you must answer- what is the required capability that my Force needs to have ( that is not a platform question, but rather a question based on you country strategic and operational planning) example I require the capability to conduct forced entry operations
Secondly, is there a change in personnel, organization, doctrine, training, leadership, and equipment needed to perform the mission. ( yes or no question) simple scenario you need the capability to perform forced entry operations. You have an fully manned, organized, equipped , trained, well led paratrooper unit, and the doctrine for conducting forced entry operations, however you have no transport aircraft. The answer is yes, I require a transport aircraft, plus pilot, maintainers, a squadron org, leadership, etc.
Then you ask what is needed first? In this case you need aircraft, because all other needs of the transport unit are based on the platform selected.
Then you examine all other capabilities that the said platform is needed to conduct, such as medevac , vip,transport, bulk cargo, etc. This will help you set the requirements for the platform as well as well as help justify cost to the bean counters. I need an AC capable of carrying 28 troops and about 10,000 lbs of cargo. But that could be easily converted to regular passenger, vip, and medevac role.
Then you examine available aircraft that meets the role and eliminate those that don't meet the requirement. You get turbo prop powered C47's, c160, c130. Etc.
Then you look at your budget and need to remember that it must also cover for spares, training,personnel, etc. So let's say your budget dictates that you can get either c130s or turbo prop c47's. However you realize that you have a bunch of near flyable c47 on mothballs or from civilians assets, and that you can refurbish the, with turbo props for about 4m ( this is actual fly away cost for a refurbished one look it up) and that there are sufficient civilian pilots and maintainers that you can use them to train your pilots and maintainers ( and as replacements on an emergency) thus making them cheaper , more accessible and able to use the remaining, money on training, spare parts, etc.
Thus this is a case in which an old plane meets the requirement.
However, if you look at the p51 mustang, you will need to ask, are there enough p51 near flyable p51 mustang airframes out there, do we still have the industrial capacity/capability to produce spares and what is required to get them flying. If we do not what would be the cost of acquiring it. Then you add training, personnel, and other cost and ask yourself is this cheaper than buying a modern fighter, the answer will give you the solution.
a side comment on industrial capacity and capability. And going to another subject for an example
Do the us have the industrial capacity and capability to rebuild the Saturn v rocket. The answer Capacity yes, capability not. We have lost the machine tools and trained personnel with the skills to bring back the Saturn v . Can we recover the capability yes, but it will cost about the same if not more than a clean sheet design ( never mind Congress). That is the reason why nations continue to build submarines, aircraft carrier, tanks, planes, and nuclear weapons. Not because we really need them, but rather because restablishing the industrial capability will be too expensive or time consuming when we do need them.