The fence and arbor (bearings with a post that fits in the router chuck) were on sale a couple weeks back (advertised with the speed increaser), because I bought them. Should be here by Wednesday. Much of the rest of the shaper package I received with my M7. The table insert is often on EBay. The dust collector (clear with lower brush) can be found on EBay, but usually doesn't include the post and sliding t for mounting to the table.
This was the first time I have seen the fence offered for sale with the speed increaser and actually saved me money because I didn't need more than the fence. The bearing arbor was an upgrade for me.
I'd have to check, but I think I do remember seeing the shaper package on sale, but don't know when.
The 500 shaper fence can be adapted to the 520 table by drilling two holes. Doesn't have all the adjustability of the one designed for the 520, but is sufficient IMHO to work reasonably well.
Regarding the SS versus free standing. First, I think much of it can be determined by the project and your style. Is it easier for you to switch back and forth between the shaped pieces and other details, or can you shape everything, then go on to the next phase? I was never successful at setting up the cutters to exactly the same height from the table (old setup with headstock above the table). Not certain that below the table will be any easier, but in my mind, safer.
Second, do long term projects have much need for shaping? Maybe the cost of a standalone is a good long term investment or maybe not. Only you can justify where you place your funds.
Third, what space to you have to dedicate?
Fourth, power to make initial cuts versus time spent make multiple passes. This of course is highly dependent upon species of wood and cutter design, however my old 510 motor was not happy making deeper cuts on oak, so I would make more passes to remove material. An acquaintance has a Grizzly shaper which can scrape out in one pass where I needed three (at least). I'm hoping the PowerPro will change that!
So it comes down to time and money.
I have 10 plus cutters which will work for nearly any design I can imagine. My biggest concern before the M7 was that the cutters were exposed from above. Additionally, space is a huge issue. Personally it made much more sense to buy a better fence and collars than to consider buying a freestanding shaper.
Not sure if any of that helps.
Be well,
Ben