I have a 2010 75th Anniversary Standard model.  It has night sights and VZ grips and disconnect removed.  I am ready to put $400 in parts into it to make the controls more usable, give the trigger a better shape/pull/reset, and add more durable internals(sear, extractor).  I know the gunsmith who will do the work, and I doubt he will charge me very much, if anything.  I am tempted though by the idea of a 2nd Hi Power, because it is the best handgun in my opinion, and I want more.  But if I buy a second one, I will then think it needs even more upgrades than the current one(sights, grips, + everything mentioned above).  Thoughts?

I would get a second one, spend the money on upgrades and refinish on it, then put your first one back to factory condition.

Too late for that, I sold the original grips(too thick) and I have always hated the 3 vertical bar sights and probably tossed them.  It has been a work gun(when I was working at gun stores and a range), so I'm not trying to keep it in pristine shape, just want it to be better.

$400 isn't going to get you very far with a second gun.  
Like into a beat up milsurp BHP.
And then you'll have 2 that aren't what you really want.

If you can do the mods for $400 and have the gun you want, I'd say do that.
Then pick up another later on down the road

Your choice. I don't think I'd want my tuned CCW BHP to be an Anniversary model. But you've already started making changes on it. So be it. $400 isn't going to go far for modifications. Trigger job, trigger modification, reliability package, new internals etc. will eat that $400 up quickly. Unless your qualified gunsmith is doing this work for peanuts. A 2nd BHP is another whole investment that will add up and quickly surpass $400 from start to finish beginning with night sights.

Modifications it is.  The gunsmith is very qualified, and a friend, so it shouldn't be too bad.

Both

Is it Alex?

You probably wouldn't know him, but he is well known in DFW.  Charles Burford.  I worked with him when I was at Frisco Gun Club.  Great guy, extremely knowledgeable about all guns, especially Hi Powers and 1911s.