I suppose you will get all kinds of opinions on what will work best. But the reality is that the only way you will know for sure what is best on that computer is to try different configurations and decide what works best. I have seen comments from users with very high-powered computers, fast processors, more than enough RAM, and yet they experience performance issues. I have a very modest HP computer running Windows 10, and I have no issues with Lightroom. I see very little (if any) difference when trying to work with images on an external hard drive. Hopefully, someone with more technical expertise will have a more precise answer for you.
I have no special technical expertise, but I can tell you what I have and am delighted with. I bought the 27 in. iMac 5K about a month ago and opted for the 2 TB fusion drive. My photographs currently take about 600 Gigs, and there is no issue at all with loading, etc. If I unlikely add an addition terabyte of photos, I will simply add a thunderbolt HD and move the oldest (and least accessed) folders there. More likely by then I will want to get get another new computer since Moore's law still holds true, so in 4 years or - here we ago again