new iMac: cable for Target Display mode and external HDD

hi,

update dual imac setup new 2017 imac.

@ moment have late 2012 imac "main" unit , 2009 imac secondary monitor (target display mode).
plan use new 2017 5k imac main unit and downgrade late 2012 imac secondary monitor (it non-retina model stills supports target display mode)
first question is: what cable need connect 2 machines?
• there thunderbolt 3 thunderbolt 2 cable?
• or need thunderbolt 3 thunderbolt 2 dongle , can connect 2 machines standard thunderbolt 2 cable? (like 1 own used external hdd)

then, need connect external ssd use scratch unit @ moment connected late 2012 via thunderbolt 2... best possible solution 2017 imac? dongle? (isn't expensive?) move hdd new case? , best connection: thunderbolt 3 or usb 3.0?

external hdd using thunderbolt 3 port "standard" usb-c connection faster usb 3.0port?

sorry, searched online still confused thunderbolt 3/usb-c vs usb 3.0 thing.
understood usb-c "form-factor" , many different connection can operate trough it, still have lot of confusion on best way use thunderbolt 3 ports @ max potential (e.g. using thunderbolt 3 port "standard" usb-c connection waste of potential throughput)

in advance help
ciao,
m

 

from point of view you'll need thunderbolt 2 cable , official thunderbolt 2/3 adapter apple working.

maybe it'd better idea sell 2012 imac , buy 4k display instead?

 

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