You don't want to create a wet area.
You can't drain your water to neighbors land or council land ect.
You could dig out the area and use type one stone and drain water to base..
Also it depends on what your paving with, block or slab ect
Technique is everything when laying slabs.
Let the slab take the weight, walk the slab, rocking corner to corner.
Place slab on edge of laid slab , slab standing up and lower it into place, adjust with trowel or shovel to get gaps correct.
Repeat
You need course sand or grit sand which is also known as sharp
If wet laying with mortar you want building sand.
Your merchant seems to have staff with no knowledge?
I'd always try and keep water away from the house if possible.
You'll probably be ok with natural drainage but if it isn't sufficient you can always dig a small relief drain so the surface water gets away to another area,soak awy or low lying safe area to drain ..
Its plastic pipe.
You wont damage it with compacted mot stone ect .
Base it up, sand cement over pipe if you want to be ott and lay the patio.
Plastic pipe is very strong unlike salt glaze and clay.
You would be better off using geo-fix jointing compound or similar.
Dry mix and damp down is very amateur and will last about a month before weeds re appear.
Wet mortar is the best applied with a mortar gun for diy and pro.
https://www.screwfix.com/p/roughneck-brick-mortar-gun/67965