I have just received my SP200 and it does what it says on the tin. It solders so much faster than my old Weller

The MX500 is a multi tool system, you can have 2 irons or 1 iron and tweezer iron. It is up to you wheter you need that.
I have used both the sp900 and ps200. I cannot tell the difference between them.
The sp900 is older and the number of tips is starting to be limited.
The ps200 is the current base model and is well supported.
The mx500 is also current and has more tips available.
At both the companies I worked at, they never had a tip wear out on them (R & d department).
The currnet company I work at keep a stock of bits for the production workers, so I guess they do wear out.
Yes, I have read about problems but I have never had or seen it myself.
One place did have to replace the iron/handle but everything was fine after that.
The range of tips is bewildering

Do not worry about the specialty tips for smt, there are loads for taking smt chips off - they work but total waste of money for hobbiests, they tend to use hot air iron.
Have a look at the tios you like on your Weller and then look for something similar
Hope this helps.