[thrash]

Jovian Chronicles has a nice inversion of the "put stuff in a box" model. You choose the capabilities first, then calculate the size of the box it takes to fit it all. There's no explicit power budget, for example, because "competent engineers would make sure the power is sufficient." Their delta-V calculations look odd, but are actually realistic.

There's also something to be said for the simplicity of Warp War, if the tech assumptions match your setting. I've often thought there's a great game to be had by combining Warp War ships, shipbuilding, and combat with the setting and map from The Company War, which models CJ Cherryh's Alliance/Union universe.

I cribbed a lot of the boats in the Serenity roleplaying game from real-world aircraft, suitably jazzed up for the setting. The rules were sort of empirically derived from those benchmarks.