Buying good marines

[Moige]

I'm training marines to get the #deca capital terraformer (or whichever it is) in X3TC's terran campaign plot.
I kind of understood the basics, and as with every other mechanic I assume I'll get the fine details once I've actually tried it for myself a couple hundred times.
One thing that bothers me is the fighting rank of marines. I understood that this can't be trained except by real boarding action, and as such it's better to buy soldiers with already-high fighting stats.
My issue is that I've scoured the entire map and found barely 15 people above 40, including one boron beast with 60.
Now every marine training facility/military outpost is choke-full of worthless soldiers, and here's the first question:

Do the soldiers on sale get better/replaced over time, or should I buy them all and dump them out the airlock so others would come by? I'm well moneyed so that's the easiest way I can see.

Also can I make my own homegrown soldiers? I'm not sure I can obtain either military bases or marine training facility/rehab facility in vanilla.
Also also, apparently you can do training at an equipment dock (?), would buying & building a dock of my own let me do free training there?

[Martin Head]

or should I buy them all and dump them out the airlock so others would come by? I'm well moneyed so that's the easiest way I can see.

This can work, as the Marines at the bases will be replaced, and you might get some good fighters.

I'm not sure about X3TC, but in X3AP, when you board a ship, some the Marines on the ship you are boarding may eject into space. Some of these can have good fighting skills.

[X2-Illuminatus]

Don't waste too much time on training newly bought marines. Instead throw them in big numbers (easier with M7M ships and boarding pods) at easy targets, like race TM and M6 ships. Those that survive with fight experience can be trained further. See also this post by Cycrow.