My opinion: The Lightning is the last ship that will set you back below one million credits, unfortunately it isn't much of a ship. While the ship sports some heavy fire power, it can easily be destroyed because of its weak shields and even weaker armor. If purchased, I recommending selling the Rocket Launcher and Heavy Rockets, freeing up 34 tons of weapons space. Upgrade the laser cannons to five proton cannons, purchase a layer of Armaplast armor and use the remaining space to buy missles. Unfortunately the lack of cargo room is a serious problem; five tons of cargo room will only enable to you complete some of the lower missions, and sacraficing twenty tons of weapon space for a cargo pod just isn't worth it. Avoid flying in a lightning.
Computer controled: Watch out for close encounters with Lightnings! The computer enjoys sending them on suicide bombings where they fly right towards you and launch a heavy rocket. If you have missiles, firing three in sucession will render the Lighting disabled. The Lighting is the first ship to come after you if provoked.