My opinion: At a steep 10 million credits the Kestrel is very expensive, but worth every penney. It'll most likely be the ship that you have for the longest time, and for a reason. The Kestrel has enough cargo room to allow for up to four mass expansions. That additional 40 tons of weapon room coupled with the standard 120 will give you plenty of fire power. Maxing out the guns (you'll have to purchase a laser or neturon cannon to do so) and turrets is recommended, as is buying another missle launcher. Tritanium armor is a must if you want your Kestrel to be engaged in heavy warfare. The Lightning-Bomber fighters are a nice touch, although they're costly to replace (1 million credits each). Having a Kestrel as an escort is great, if you're able to capture it.
Computer controled: You should only go up against a Kestrel if you're in the same ship (or a frigate, destroyer, or cruiser). The computer Kestrel will launch its Lightnings all the time, giving you plenty of heavy rocket pressure. You're better off battling a Kestrel in close range — its two proton turrets aren't too powerful.