Graham, I understand your frustration and possible annoyance at having to come to your decision.
The modelling lark is supposed to be fun and when any form of pressure creeps in it could be time to step back and consolidate.
However, there is still time enough to produce something. So why not just opt for one vehicle and a couple of figures?
The diorama doesn't have to be complicated, simple is sometimes best.
My friend, you say you can't get it right. To my way of thinking the only way to see where you can improve is to produce something, anything. Post it, and being the sound bunch we are, you will get all the help you could wish for.
It doesn't matter if in your eyes it's not good enough. We are not here to belittle folks work, only help.
We have all tried different aspects of modelling and been well 'Outside our comfort zone'...(Don't you hate that expression?!) so I say, simplify it, keep at it and finish it.....
'nuff said.
Ron