The trouble is that if you spend too long considering it very very carefully, somebody else jumps in and buys the player. And do you really think that a big name would accept a trial or allow themselves to be put through one of your "'stringent' fitness tests"? They more likely to say "bolllocks to that" and sign for somebody who has got more realistic views on the signing of "senior players".
The thing is about "senior players" - they hold all the aces. The ultimate goal of your scheme is only to sign fit senior players - the ultimate result would be that you would never sign any. If you think that senior players are an essential part of the side, you've got to be ready to take a risk.