Fusion power plants are one of the best structures in the game. Compare the three +production buildings:
Manufacturing Center: +25% prod, 100 cost, 1 maint
Fusion Plant: +20% prod +10% econ, 50 cost, 3 maint
A-matter Plant: +30% prod +5% econ, 300 cost, 1 maint
Very early on your worlds will have a base economy of 20 BCs, which means that a fusion power plant really only costs you 1 BC in maint and an A-matter plant is free. In the mid-to-late game you'll easily make a -profit- with both the fusion and a-matter plant (usually +1 net BC per turn with fusion and a-matter by then). To figure out the true maintenance/profit value of a building, figure on about 20 BCs for a planet's income in the earlier part of the game, and about 40 BCs for the later part.
Because it builds so cheap (twice as fast as a manu cntr), my build order is fusion plant, manu cntr, then a-matter plant. You'll pay a bit more in maintenance (but only for a while), but most importantly you'll ramp up your production capability much faster.
Corvettes are the best ship you can have for a turtling game until you get to a-matter missiles. Because the AI only calculates attack strength when it determines your military strength the corvette has the best attack-to-build-to-maintenance cost of any ship except the a-matter missile. The computer currently thinks that 2 corvettes (100 build cost total) are 20% more powerful than a battlehammer (200 build cost), and the two cost 33% less maintenance total as well. Just don't let an actual war happen though or reality will creep in (P.S. I've posted this as a problem in other threads and asked that the military power algo be fixed).
Stellar Cartography is most useless for me, because I play on abundant planets and I use scout ships, which makes that tech totally worthless.