Like space cow said, if they have lots of mass driver weapons, make sure your designs have armor to defend against them. If speed is not an issue, skimp on the engines a bit, just enough to move the ships into defensible positions around planets.
Focus on a weapon for which your enemy does not have any or very good defense for. In this case, if you have focused on missles and your enemy has high anti-missle tech but no armor or shields, you might focus on mass drivers or beam weapons.
If your espionage is high enough, click around their ships and fleets to examine the weapon platforms and then design specific ships against those threats. Build some fast transports to get through any space naval forces once the orbital naval forces are taken out and capture high production worlds to reduce thier turn around on rebuilding thier fleet to counter yours.
Try a mix of designs based on the threats and needs for your pending invasion.
Above all, try to stay friendly with them for as long as possible while you build up your military... unless they shoot first in which case, good luck.