You said it yourself, the Sten isn't practical or ergonomic, and the CM16 is recommended by everyone. For a starter gun, you really aren't gonna want to spend anything above, say, £200. G&G's entry-level M4s are so highly recommended for a reason. They're reliable, they perform well, they're easy to customise, cheap, well-built...the list goes on. Personally, I'd suggest that you take the advice of so many other players on here...
On the other hand, look at this way. Airsoft is a constantly growing sport, and new innovations are constantly being made. In two decades, we've gone from TM'S Famas to a ridiculous number of different models. For your first AEG, you want to make sure that you're happy with your purchase, because the chances are you'll be using it a lot and for a long time. Why limit yourself to three choices?
There's so much out there to choose from. This forum is a bank of knowledge, so use it to your full advantage! If ever you need a more specific idea, feel free to hit me up.
Happy 'softing
Theta