Powering SBC and a 3.5" HDD from one PSU - with Spin-down (hd-idle or hd-idle)

dear friends , fellow pi-experts, 

i have some old 2.5 and 3.5 HDDs here - its still slightly cheaper than buying new ones off-the-shelf USB HDD drives of the same capacity. So - for the NAS i will use the old  ones - and I get to select the HDD of my choice for that enclosure.

i use Armbian on several systems - bananapi - udoo-quad  - orange pi and others more. I love Armbian!

plan; building up a NAS based on a Raspi 3 with a 3.5 " HDD.

I just want to set up a samba server in linux (raspbian, armbian, or in debian, in opensuse; here we can do 95% of the config in the graphical YaST tool, only for smbpasswd the console is needed iirc), here no monster software is needed and we learn so much more about linux ;-).

on a sidenote: i want to learn some Linux-methods and processes (eg. while using spin-down with hd_idle and hdparm see below)
btw: I like that approach, i love to start from what i have at hands, see how it works and then start fine tuning to reach the next level :-)

Assumptions; I think that i also get to select a SATA to USB interface of my choice for the enclosure as well.
I don't know what kind of brand/type of HDD and/or SATA to USB interface i need here - to be installed on those of-the-shelf units.
Do you? As most off-the-shelf USB HDD drives don't/never publish what HDD maker/type as well the interface type in their drives.
i will have a closer looks at the drives next week - then i will post the types here and let you know what types we have here: 
(note ive got several).  
  

my psu:  Mean Well RT-65B AC/DC-Netzteilbaustein, geschlossen 64 W  ( https://www.shorturl.at/vPRU2 )
  
voltage: input max / Eingangsspannung (max.) 264 V/AC
voltage: input min / Eingangsspannung (min.) 90 V/AC
number & inputs: / Anzahl Eingänge 1 x
number & outputs / Anzahl Ausgänge 3 x
Power/Leistung 64 W
output: Ausgangsspannung 5/12/-12 V/DC
Ausgangsstrom (Details) 0.5-8/0.2-3.5/0-1 A

note: what is aimed; its all about experimenting and doing some tests. 

question;
- So is this possible? is it safe?

again; I got an meanwell psu, and i was wondering if i could power my two 3.5 HDDs and the rpi3 with it?
do i need to do something to protect the rpi3, or can i just plug it and and its good to go?
The 5V rail on meanwell  power supply should be stable enough to power the Raspberry Pi without problem. 
Note, however, that most switching power supplies need a minimum load to function.
i guess that the safety of the whole project depends on the experience and on the care we take when connecting things. 
As these factors are difficult to ascertain even by by an experienced actuary, i guess that the usual recommendation is to buy a power strip and to use it. 

question: i need to have a SATA-USB adapter to attach a single HDD or two to a raspi.
 
Well I can say here is that some of my friends had a NAS running 24/7 in such a configuration for quite some time without problems.

on a sidenote: 

- 2.5" HDD only need 5v. 3.5" (and optical drives) need both 5v and 12v. 
- btw: some USB-SATA adapter have a socket to accept 12v but i'll need to make my own cable.

Regarding the power consumptions: 
- the whole world is full of stories about the "power hungry" machines: 
- But they sit idle most of the time with a few watts....

so that said - i want to use some special methods to spin down the HDD - with HD-parm or -Idle...

btw: I don't have control of what brand/type of HDD and/or SATA to USB interface installed on those of-the-shelf units.
Do you? I guess that most off-the-shelf USB HDD drives don't/never publish what HDD maker/type as well the interface type in their drives.

-....regarding the spin-down with some linux-techniques: 

a. hd-idle:
b. hdparm: 

....see a hd-idle: http://hd-idle.sourceforge.net/

Quote

hd-idle is a utility program for spinning-down external disks after a period of idle time. Since most external IDE disk enclosures don't support setting the IDE idle timer, a program like hd-idle is required to spin down idle disks automatically.

Power management configuration :: https://wiki.archlinux.org/index.php/hdparm

Quote

Modern hard drives support numerous power management features, the most common ones are summarized in the following table. See hdparm(8) for the complete list.

i am pretty sure that i can use hdparm & hd-idle to learn some more linux and to do some experiments with spin-down...
How would you use these two processes - on event or how !?

note  -  on the images beslow you see various setups - with raspi / banana pi and Udoo quad. 

but i have a bunch of raspis here - raspi 2 and raspi 3 - and yes: i want to go the raspi 3 way...

note  -  on the images beslow you see various setups - with raspi / banana pi and Udoo quad. 
but i have a bunch of raspis here - raspi 2 and raspi 3 - and yes: i want to go the raspi 3 way...

btw: how you like the mechanical - setup!?