Upgrading an HP Z1

Trying to help the son get his desktop computer in a little better 
shape.  He's a 38 year old hard working guy who is trying to make it on 
his own, which I really admire.
The core box is from this class

<https://support.hp.com/us-en/product/setup-user-guides/hp-z1-entry-tower-g5/model/28722929>

This computer is mainly used for CAD type work, specifically SolidWorks, 
that he uses to program files to use on his Doosan CNC machine.

I found a few things when looking at it.
First, he only had a small 256 GB SSD drive, with no backup drive or 
additional storage capacity.
It looks like it came with only 8 GB DDR4 ram, but was immediately 
updated to 16 GB.  I'd like it to go to at least 32 to speed things up.
Power supply should be big enough at 250 watts, though it does have a 
dedicated video card which the SolidWorks sales guy recommended.

First, it has the 2nd slot available for a 1TB M.2 SSD.  I figure I 
could copy the 256 GB image to the new 1 TB drive, and erase the 256 GB 
and use for storage once I've switched the connections for booting and 
am sure it works.

There is a slot for a SATA spinner and appropriate cables from the power 
supply.  Not interested for now in an external drive, because for him 
time is an important factor.  I don't want him sitting around waiting 
for files to be copied/written from/to a USB drive when he could be working.

It is rather intimidating looking for hardware, especially when I see 
things from very knowledgeable people here talking about problems and 
flaws I wouldn't even know existed.  So my questions.

The new SSD should be straight forward, but I'm wondering if there are 
things in particular I should look for?  I think I want an 1 TB M.2 NVMe 
drive.  I know I have to check to make sure length and width are the 
same.  Anything else regarding capabilities I have to consider?

Ram should be straight forward for the most part.  Biggest question is 
if the board will recognize two additional 16GB sticks, or if I would 
have to settle for additional 8 GB stick?  I think it came with 2666 
MHZ, and I see you can go faster.  Would mixing the two speeds just 
fallback to the 2666 MHz speed meaning it would be useless to get the 
faster ram?  Also, the CAD is very resource using things and I see one 
of the sticks in it has the heatsinks on it.  Are these really needed on 
Ram or are they hype?

Installing a SATA spinner for backups has me wondering if 7200 rpm drive 
is necessary, since most of the backing up will be middle of the night. 
How much difference does cache make?  I've seen cheaper ones with 64 MB 
and what look like better at 256 MB and I believe expensive ones with 
512 MB.  I also figure to get him current, I should be looking for SATA 
6 capable drives?

Any and all advice, as well as things to either look at or stay away 
from really appreciated.

TIA
Sticks


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