There are a lot of implications, for instance with 100 T-54s you need four hundred men for crews, and the cost of moving 100 tanks around would probably make newer tanks cheaper.
Of course you could argue it would take 100 shots to take them out at the minimum but it really depends on your situation and who you might be facing... modern ERA and APS systems might actually render them as effective as better armoured vehicles.
New types of ceramic armour could potentially be fitted to improve protection without increasing armour weight, so a new more powerful engine might give it impressive mobility.
Having said that a wheeled vehicle like Boomerang might give you much cheaper operational and maintenance costs with better fire power and comparable protection.
Uran-9 with Kornet missiles and Bulat missiles and Ataka for surface to air use might work out more effective.
It rather comes down to what you already have and what you think you need.
You can upgrade old stuff forever but eventually it reaches a limit.
I would suggest for example a poor country with lots of T-55s might consider buying upgraded T-72s and also apply the upgrades to the T-72s to their T-55s as well for standardisation and performance improvements across the board.
In terms of fighter planes I would approach an existing user of MiG-29s and perhaps work out a deal where you buy their old upgraded MiG-29s and they can use that money to buy brand new build MiG-29M2s.
Continuing to use old aircraft only makes sense if it is still being used by the original owner. Otherwise you fund all development yourself so the next generation product up is what you should be looking at...