In terms of ease of obtaining parts, my brother in law lives on the west coast of SA (most people would call that pretty remote) and in terms of basic parts for servicing, he's so far had about a 50/50 strike rate with the parts he needs being readily available through Mitsubishi Oz, because the 6A13TT is a sibling to the 6A12 motor of the Galant hatches sold here in the mid '90s. For the rest, one phone call to Mitsubishi NZ solves the problem.
I actually owned my Galant BEFORE they became eligible under SEVS (brought it back as a personal import in 2000), and just bought all my parts through Mitsu NZ, three days transit time if I was ever in a hurry and needed them asap. Most times, I'd book in for a service at my local, and my mechanics would contact the parts guys in NZ, they'd list the cost for all the parts needed for the service, I'd pay on VISA, and they'd turn up a few days later. It's even easier now because there are workshops who are starting to get more and more experience with them.
As has already been mentioned in this thread, they're not great for extracting huge hp out of, mainly because it's pretty tight working in the engine bay. The auto gearboxes work fine forever if they get regular fluid changes, same with the AYC diffs.
In terms of size and seeming roomy, the Stagea is 30mm longer inside, but the Legnum is both wider and taller than the Stagea, as well as being 80kg lighter...