I don't have a world editor on this comp and I doubt giving you JASS will help since you wouldn't be asking this question if you knew JASS, so I'm going to give you a rough sketch of the GUI triggers, I'm not quite sure of the exact text though:
Event: Unit sells a unit
Action: Unit - Issue Order (manipulated unit) to right-click (buying unit)
If you want them to continuously follow them, then you will need to make a timer that goes off every x seconds, as I don't recall there being a "follow" string command. However, depending on the AI and what you want exactly, this may just work fine alone.
For the summoning event, in order to make it clean, I would recommend JASS, but since you didn't really describe exactly what you want, here's the idea
Event: Unit casts an ability
Boolean: Ability equal to whatever ability
Action: Unit Group - Pick Every Unit in (entire playable area) equal to (the summoning unit) owned by ( Player who owns triggeringUnit ) and order (picked unit) to follow (triggeringUnit)
You can also add the ability locust here in the action, or just give the units locust in the object editor (recommended). Note that these triggers are really ugly because you're still being very vague!!!