Page 1 of Outdoor (Deck) Lighting - Electrical Advice

Having a 12ft.L x 20ft.W deck built for walk-on access from patio & garage door(s).
We intend doing two things `Electrical`

1) Put 16 Low Voltage IP68 1W led mini-lights around the perimeter & in the steps.
The transformer from the deck-lights supplier is a PLD 1-16 350mA constant supply
(upto 16 luxeon led via 2 series circuits + 8 LED each)
Also intend buying 18(2 spare) mini waterproof `jelly` connectors
which will require `x` amount of CR135 1 x 0.35mm teflon cable

2) Put an outside dbl-gang socket on the external garage wall
(primarily to provide source to Xmas lights at bottom (20ft away) of garden)

I will be getting an electrician to sort this out
Are there any issues I need to be aware of doing this, in particular

Q1) Would it be easier for the Electrician to `put something` down e.g. `conduit` before the deck is built?

Q2) Do I have to buy `their` `dimming` transformer which limits me to 16

Q3)Is it best to house transformer in garage (and cable thru garage)?

Q4) What is best regarding the cable to power the Xmas lights at bottom of garden
(run it under deck?)

Q5) Anything else that needs taken into account?

Cheers.
Ron