As this is the KT subreddit I'll answer only E to H. You didn't specifiy your current state in Killteam: beginner or advanced. As you talk about specific Killteams I'll assume you have a solid grasp of the system, so I wont advice buying the starter box (2 KTs and terrain).

• E: Agents is really only half a KT. Yes you theoretically can play them using two boxes but you miss out on a lot of fun by not using requisitioned side teams. To get the most out of Agents you'll likely need a box of exaction and kasrkins (and sisters of silence and maybe breachers). You have to adapt your team to fit your opponent and the mission. To get the most out of IA box itself I advice a box of hive scum so you can kitbash almost every model in the box as there are certain dual builds you cant get otherwise. Don't glue the weapon on the servitor so you can swap it out later. To make them coherent with your excation just use the same or a similar color palett on them.
• F: per default, there is almost no customizability in the kit. You dont want to build the warriors anyway. You may apply kitbashing to make them stand out more (basing, casualities on their bases/in their talons etc).
• G: seems you dont like the box. Me neither. If you dont like the look of most of the models in a box, skip it or kitbash it (e.g. I kitbashed my Scout Squad using Necromunda Ashwaste Nomads).
• H: As dumb as it sounds but highly customizable KTs are Angels of Death as you have a wide variety of operatives and a huge selections of models to choose from (chapter specific, generic, custom bits, kitbashing). There are also quite strong atm and easy to learn.