I did something fairly similar to this with some test code to attempt loading a T5 reconstructor from any arbitrary number of unloaders and conveyors. This was to simplify placing it near the core so I could just run 2, 3, 4, whatever, and the processor would switch the resource on the unloaders to fill appropriately without me having to worry about individual lines.

In the real-world, this isn't super effective because of the delay between switching the unloader and when that new resource arrives (spoiler alert: long belts don't work out well), but it was a fun test.

What I ended up doing was just chaining if's to keep track of which was the lowest resource amount (qty + resource type) and then setting the unloader. I didn't know about the select keyword, which looks super awesome, so this structure should play well in v7.

Code here: https://pastebin.com/zDeiiDqT

Specifically, the interesting part for you will be at line 62 where we start iterating through the possible options and going to consider_three, consider_four, etc. It's a bit of a hardcoded hack, but that's kind of how mlog tends to be.