I like to compare it to traditional MMO's- considering time you invest into reaching ship tiers. Lets say the Python is a level 60 and the viper is level 20 - 25 ish.. then the viper would never be able to take out the Python. 10 vipers would be able to take out a "bad" level 60 python though and that is just fair. As it stands now there is no balance between time invested (i.e levels when compared to standard MMO's).
If there was a viable non-space trucking way to get the necessary credits then maybe this could be considered, though at the same time a number of twitch based full PvP MMOs (Mortal online, Darkfall) allow for even a novice to successfully kill a skilled up player in better gear (time spent doesn't necessarily give you an overwhelming advantage) so I'd still disagree with your approach.
Second, if your approach was followed then the vast majority of players would simply grind up to Pythons/Condas and start PvPing then. This would be negative in that all real PvP would be focused and limited to higher tier ships and you would mainly see high end player ships (no diversity or use for smaller ships)
player skills should always be the final arbiter in a twitch based combat game.
splashing out for a bigger ship should give you an advantage (more shields, hardpoints and hull strength) however it should always be possible for 2 smaller ships to use their best points to take out a larger one.
every ship above a sidewinder type needs a role and should be useful.
personally I hate the WoW mentality that only top tier PvP is important and that everyone should race to get there, I much prefer a player skill focused approach that rewards time investment only to a limited point.
If you're a bad player you should die to a single viper/cobra, your time spent should at best but you a chance to escape but not give you an overwhelming advantage.
player skill should determine who wins, not who spent the most on their ship.