You don't necessarily need more than 200Mil for a decent trading conda, if you know what you are doing a little bit.
First, 5A shield, some A0 shield boosters and 8C powerplant, also D thrusters.
Then fit it with cargo racks, it adds up to 436t.
Then, for weapons, fit some mine lanuchers.
When interdicted, it's mostly by large ships, I don't know why but whenever trading in my trade conda, I only get interdicted by deadly pythons or dangerous anacondas. Sometimes they go up to elite but the real problem comes from small elite ships as they tend to have multiple rails.
Anyway, when interdicted, set a nearby system as the target from the left hand panel, navigation tab. Anyone will do.
Then, full pips to shields (system) two to engines, deploy hardpoints and start dropping mines while boosting away towards your target system WHILE THE FSD COOLS DOWN. The mines will either directly damage the attacker or they'll at least push him around and cause him to miss. Then, as soon as the FSD is ready, retract hardpoints and don't wait until they are fully retracted. You can just hit jump as soon as you hit retract. Keep boosting until countdown.
I haven't been killed once in my sub 200mil anaconda while trading and I've been interdicted since 2.1 launch upwards of 30 times in that ship. Always dangerous and up NPCs.
Or don't try this out and keep spreading horror stories and scaring yourself off. The only thing you are doing by spending more than 200 for a trade conda is increasing your rebuy, thus your risk for the same profit in the end. This is also why I prefer trading in a t9 to a conda and I don't really understand trading in a cutter. The extra profit is not enough to cut it with the double rebuy over a conda.