I tried several "unofficial" sources and it's official : you can't get a new TRF511 kit at the moment, not even the expensive kit with the gear diffs.
Second hand : everybody seems to hold on to their 511's at the moment.
What worries me about the 502 in terms of reliability is the front shock tower assembly. We race a lot on astro and i can see troubles ahead with front gearcase breakages and front shocktower cracks (my 501 front shocktower broke regurarly untill I started using a 3racing one. That never broke again). And the A-parts seems to be difficult to obtain and they are expensive. Also the new alu steering assembly is expensive.