I think your proposed plan is workable, but you will need to keep a close eye on what your are installing on where....(it's easy to become confused!)
I would be inclined to consider doing it this way:
1 Download and install Macrium Reflect free (disc imaging application) and take a complete disc image of your existing C drive and store the image on your new hard drive that you have put in your external caddy (no need to do any partitioning at this stage, just leave the new hard drive in the external caddy as one big partition.)
2 Use the "wizard" in Macrium to create a bootable CD...(useful in the event that for some reason in the future Windows will not load... you could then use the bootable CD to boot your computer and access any disc images kept on removable media...) This step (No 2) is not essential but nevertheless is a wise move)
3 You can now remove your C drive from your computer and replace it with the new hard drive that's in your external caddy (Keep the C drive safe!)
4 You should now have access to the "new drive" which has your old operating system on it together with all updates, applications, and data files etc.NOTE:
if your image of your C drive contains any bad code, (Virus, Trojan, Worms, unwanted material or just junk and detritus accumulated over the years then it will all be faithfully reproduced onto your new drive!..... might be well worth your while checking and double checking that all is well with your C drive, prior to making the disc image....
5 If you want to now partition your new drive then proceed accordingly...
6 You can use Macrium Reflect free to create extra images from time to time, as a part of your regular back up strategy, but do keep several copies...and with at least one kept safely offsite. Tip:
Make sure you fully understand all the instructions on using Macrium BEFORE you start..... I have known folks become confused and obliterate their operating system completely by getting confused and overwriting it!
There are of course other disc imaging applications, including free ones so check around and make sure you are 100% comfortable with the one you select before starting...
As these steps involve working inside your computer then make sure before you start that you know, understand and can apply correctly, all the personal safety and ESD precautions that are necessary...... If you are in any doubt on this at all, it would be wise to consult a local competent computer technician, and ask them for a quotation for them to do the work for you. It's good to save money, but not at the expense of your own personal safety or that of other members of your household.