Propulsion:
1.0 power always feels wrong to me. Go above and costs/manpower required scale linearly (1.2 is 1.2 times as expensive). Go below and costs scale quadratically (0.8 is 0.64 times as expensive).
Larger engines are more economical.
For a fixed engine size, a 5:2 split between engines and fuel is the most weight-efficient. You usually want to err on the side of more (and less stressed if you don't need more speed) engines and less fuel, unless your vessel is intended to routinely refuel others.
Mesons are dirt cheap, but among the weakest weapons per ton. As such, they are a good match for slow ships - at faster speeds, moving bulk is expensive and the cost of the weapons is less relevant. Dilemma: turret gear and sophisticated fire controls are expensive. If you're going for cheap, you may prefer to go all the way - commercial engines, no turrets, no armour since that's more expensive than cheap components per HtK, simply build more ships.
However, for point defence on a budget, it's hard to beat 10cm low-velocity railguns.
If speed is not a major consideration but PD capability is, a good alternative to turrets is deeper research into BFC tracking speed than other military techs. For unturreted weapons, tracking speed is capped at the higher of ship speed and standard BFC speed. Similar to the power multiplier issue, sticking to "the defaults" is subtly discouraged - with 3000km/s base tracking speed, you could go faster and enjoy better TS without turrets, but you aren't penalised for going slower. Same ship speed as the base BFC speed feels wrong.