Welcome to the starting of the "yellow brick road" for 1911's and the forum.
The RIA is an excellent pistol, I currently own a RIA 1911 9mm Tactical and it is great. The STI frame is actually made by RIA so it could be argued they are the same pistol. It is true STI have the frames shipped to them and they finish them. So, is STI and RIA the same pistol other than "human hands" finishing it?
As to the Springfield, I owned a Springfield Mil-Spec and it was fine. No problems with anything other than the trigger out of the box; it was terrible. I was able to do a trigger job on it and after that it was better but off it went. I don't miss it.
That only leaves the Para and that one I cannot comment on because I have not owned one.
Ahh the Ruger SR1911. This is the one from your list I would recommend. Why? First off it is American made! It has every option I think should be on a 1911 and NO more; Combat Trigger, Combat hammer, Beavertail safety, NO front slide serrations, SS finish, Novak adjustable rear sights, nice grips and a nice trigger out of the box. Not too mention the company of Ruger standing behind it. And all for well within the price range you are wanting to spend. There is a reason these are difficult to find and the others are not.
Good Luck - I hope this helped.