From your comment im going to assume you are newer to cars in general, however correct me if I am wrong.
As far as the 3G platform goes, the montra of "money can get you anything" is really the only thing you need to consider. It is absolutely possible to put boost on the 6G72 engine, however the internals of the car are reportedly not that capable of much boost, so check with others on the forums for those limitations. You are correct, turbo manifolds will need to be fabricated by either you or a shop. But it can be done! If money is not a concern I would respectfully refer you to take interest in club 3SI, you'll definitely get more attention with a 3000GT, as well as power if you are a dedicated mitsubishi fan.
For easier avenues of power, you are going to realistically have an easier time rigging up an engine harness and swapping out the 6G72 for a 6G74 engine, a 6G75 engine, or a 6G75 MIVEC engine which is generally considered to be the most difficult swap to undertake, but will provide the most power out of the three. Be warned, it's a bit of a rabbithole. Otherwise, you can utilize some 6G74 parts on the 6G72, such as the intake manifold and throttlebody swap, which has been reported to provide a better midrange in exhange for some top-end torque. This upgrade bascially just gets you more air into the engine. TUNE TUNE TUNE! Don't make the same mistake as many who got into the eclipse platform, did some crazy mods, didn't tune their engine, and then blew it up and blamed mitsubishi. There are a few reputable tuners on here, I believe some are pinned on the homepage.
Bottom line, money can make anything possible with this car, however you ARE choosing a relatively limited opportunity for a faster car, but smiles per gallon are determined by the driver.
Good luck!