Simple decking 2.4x4.2m - existing paving and DPC

[gambo]

House from late 80s, with paving (probably from 80s as well) cumulating water in the middle. I want to replace it with a decking 2.4 wide x 4.2m long. I've looked at the bricks and there is something like a black material between bricks at around 16cm high from paving so I assume it's DPC.

1) Shall I remove the paving or use it as support and put DPC below it?

2) Wall plate -  how much space should there be given I won't be under DPC with the plate, 

3) Is there a good tool to plan it? e.g. how to optimise number of decks etc

4) What joist shall I use? Does it really need to be 150mm height joist? I would prefer it to be lower so that it doesn't add to much height.