About to do this very soon on a late 60s built house with enclosed
garage.

I'm going to get a UPVC window to fit totally across the top half, to
existing brick, and use new bricks/blocks below, which will be rendered
with concrete.

I'll also stick a DPC at the same height as the existing one.

Do I need to do anything else with the DPC and can I lay brick straight
onto the concrete floor with mortar or should I prepare it some other
way?

Any other advice appreciated.

Cheers.