This is a bit late, but better late than never, I guess.
The basic problem with mech collecting is that the base skull rating of all planets increases as you advance the storyline; by the time you get to the end, even the starter systems (with the lowest difficulty) are 3 to 3.5 skulls. The actual contracts you get from a planet have a random spread around the base rating, but (at least in the 1.0 version of the game) this isn't big enough to make a significant difference.
The 1.1 update makes mech collecting much easier, however:
- The spread of difficulties is increased from 0.5 to 1 skull about the base
- There is a separate difficulty level setting, which controls the base skull rating of all planets.
So, what I did was set the difficulty level to "Easy" which reduces the skull ratings for all planets by 1 skull from the default. Then I went back to the starter planets, which were now rated 2.5 to 3 skulls. The actual contracts on offer went as low as 1.5 skulls, but anything with 2.5 or less was usually populated with mediums and lights. After just a couple of hours bouncing around the starter planets, I finished my collection.
Some planets where specific mechs are always for sale:
- Cate's Hold (Magistracy of Canopus): Shadowhawk (both versions)
- Panzyr (Arano Restoration): Griffin
- Smithon (Arano Restoration): Dragon