Is this going to be a Historic Adventures project? Check out some of the fleshing out I've done there for my "Corporate Menace" adventure, which takes place 100 years before the UPF.

Exactly how far before the first war is this going to take place? I would say that atomic drives came to be around that time (first war), I don't have them in my pf-100 game. If the subspace radio isn't developed yet, it has to be more "ancient"...as I would place the subspace radio and ion drives in a similar era (but not neccesarily in the same generation either, the ion drive would predate the subspace radio).

That said, it would be more of a Traveller-like setting, with information and messages being sent via starships instead of subspace transmissions. You would need an established communications service (like Traveller's Scout service or our current Postal Service on a wider scale).

Robots and computers could still be very canon, although with greater limitations --- I increased function points in my pf-100 game as well as establishing maximum values per mainframe (meaning a super computer would be more than one mainframe unit, it would be several linked together). As such they get more expensive for what little they are by the more modern game setting equivilents.

An air car would still be doable, just probably not as fast as the current renditions and certainly more expensive (adjusted for inflation, wink-wink).

Ships --- as I hinted at, ion driven only. Slow, but you can tweak the KH rules to allow for military warships. Perhaps not as well armed as the canon atomic boats mind you, but a "prehistoric" frigate could be HS:5, 30 hull points (lighter armor than the modern one), ADF:1, MR:3, lower DCR (say about 50 or so), and more streamlined weapons like a laser cannon and battery along with a single rocket battery (w/several reloads) or you could develope more vintage versions of those weapons that cause less damage r shorter ranges...in my pf-100 game I have three levels of laser cannons, one for fighters, one for sub-capital ships, and one for frigates and larger each doing a little more damage with longer ranges.

As such a typical militia ship could be a HS:3 scout-type craft boasting two ion (and two chemical for atmo-use) engines armed with a "lesser" laser canon (think pod laser-ish) and a lesser laser turret (shorter range version of the battery). Fighter craft, like in my pf-100 game, would be limited to multiple chemical thrusters with varying maximum velocities (depending on fuel supply) and armed with lesser laser cannons and mini-missiles (something that does d5 or d10 tops with a slight damage table modifier to permit possible double damage)...one of my pf-100 weapon systems is a "FFR" (forward firing rockets) which is essentially a rocket battery salvo that is set up as a FF-only weapon: 2d10 damage and short range, and you only get one shot.

I'm not overly fond of Zeb's Guide...nor do I have any qualms stating why. 

My SF website

I just posted a SW1 Era Frigate in my SFKH design project that might fit the build well. Its equiped with an atomic drive with a large shield disk between the command module and drive module. It could pretty easily be adapted for use just prior to SW1.