1. A CPU with faster clock rate will execute faster, so a 3GHz CPU will be faster than 2GHz one.
2. Multiple cores require parallel processing both at the OS level and at Application Program level. If you have 16 cores then OS will use 1 core normally and the other 15 can be used by the application. Both LR and PS utilize multiple cores when needed, but most of the times they use one or two. Stitching panoramas and image processing programs use up all cores available, so if you are heavily into panorama stitching and your application can use them, the more cores the better. Otherwise if you have the Task Manager open, you will find that most of the time only a couple of cores are used.
3. More RAM will benefit, if and only if the programs use them.
. If you have a few programs open, the OS will try to retain the memory state of each one so that when you switch there is no time lag. For example if you have 5 programs out of which 4 use 1GB RAM and one 6GB, then you will benefit with 10GB or more. The system also uses a lot of RAM, so over and above the 10GB an extra 2-4GB for OS should be factored in.
. If an application can use all the available RAM, then extra RAM helps. On the other hand if an application is limited to say 8GB TAM, then having 64GB is a waste of money.
. If you have excess RAM, say 32GB or even 64GB, you can use that as a temporary storage area.
4.
. Hard disks are fine for storing large amount of data as their prices are quite low on a per GB basis.
. SSD are more expensive, but are upto 10+ times faster, so are used where a lot of data is transferred to and fro disks - OS, cataloging etc. Beware that the speed up also depends on both the SSD speed as well as the bus used to transfer data. Older motherboards may not be able to support the speed the SSD are capable of.
. A new storage media - NVM (Non Volatile Memory) has come up. It is much faster than SSD, but requires special support at MD level. Here is an article on comparisons
https://photographylife.com/nvme-vs-ssd-vs-hdd-performance
5. There are various methods of using older HDD as external storage devices.
. The simplest is to buy a "USB Cage". For larger disks that will come with an external power supply but with smaller Laptop Size disks there is no power supply needed. Just fit the HDD in the cage. Connect power supply (if needed) and connect the cage to the computer using USB cable. This is same as using a normal USB disk available in the market.
. If you have spare slot and your HDD are SATA, install a SATA card and bring the SATA and power cables out from the desktop. Connect the HDD to the SATA and power cable and you can access the external HDD.
. If you have a lot of older disks then you can get an external HDD cabinet with power supply and LAN interface. Put your old HDD in the cabinet, connect the cabinet to your computer using Gigabit Ethernet and you have an external storage.
. Lastly you can do what I have done. I have installed a 4 bay hot plug SATA cage in a 3 bay space in the cabinet. Normally it holds 4 HDD, but when I want to backup, I take one HDD out, put the spare in and read/write to it. It is a hot plug unit - you can take out and put HDD which computer on. I have a stack of old HDD - 80GB, 160GB, 300GB and 750GB after upgrading over time and now use then to store copies data. Every six months I just insert them one by one to check the data. It is now 5 years since I started the process and the HDD used for archiving are all healthy.
By the way, I have found that my 7 year old dual Xeon MB has been able to keep up with modern machines, all these years, in terms of flexibility and throughput. Only now that 4 cores I7 are becoming common, that I find that my 8 cores are feeling dated. If I build a new system, I will again use dual Xeon configuration, but by that time 8, 12 or even 16 cores/CPU will be in the market.