List of responses incoming!
First off, MOC's (like making mechs and shit) will require a pretty large base of pieces. Sets will help build it like mrklaw said, but Bricklink will flesh it out.
For powering your sets, the new Power Functions with IR receiver and such are boss. You really should buy one of the new Technic super sets (Unimog U-400, Bulldozer, etc). They really show you how to incorporate some awesome PF stuff into your builds. Technic + System sets get you a huge selection of bricks and gears, both of which you'll need if you want to make something truly fantastic with motorized parts.
Space theme is sort of on a 1950's tangent at the moment (alien abductions like WOAH), so you'll ever have to pay after market prices for older stuff, grab the Shuttle Expedition, or wait it out.
For pirate ships, PotC has a few (two I think), and you MAY still get lucky and find an Imperial Flagship for minor markup, but don't hold your breath on it!
Creator sets rock, and the Lighthouse is an excellent buy. I always suggest the houses in the theme too, but that's because I've got a soft spot for houses/architectural builds.
The Tower Bridge and Modulars are fantastic AFOL pieces. If you're unsure about them, do some searching for them on display. They are both excellent excellent additions (and have crazy after market pricing...not like Star Wars, but still 200%+ resale value). Tower Bridge is a display model in every sense and will go the way of Taj Mahal, Eiffel Tower, etc soon.
Long story short, do some searching on the Lego S@H and get some ideas. It sounds like you want to power things, so grabbing a few of the higher tier Technic sets is probably a must, and various parts and pieces for using the Power Functions are pro, too. You may not need it, but pneumatic stuff is available on Bricklink (not sure if they're in sets anymore), and they make some great hinge/piston style assemblies.
Here's the important part about your nice chunk of Lego change...
Buy what you like and what makes you go WOW.