I have a Mark V I bought new in December of 1982. It has been used and maintained since then...but it has the single bearing quill and didn't have the larger tables that the 510 has nor the better fence that the 520 has. I was in your quandary except, I couldn't begin to justify the PowerPro headstock (and still can't).
I found a model 510 with a bandsaw on Craiglist, the guy was asking $950. I looked at it and thought it was poorly cared for but not out of the question. Well, compared to some of the restorations you see here, this machine was nearly mint. I hemmed and hawed and got both for $500. I don't think this was an exceptional deal, but I'm happy with it. The tables were stained, there was a little rust on the support tubes, and a few "scars" from use. The short version is, the machine cleaned up well and looked new in about 2 days and I set it up correctly, probably for the first time. I sold the band saw for $135 and still plan to sell my old Mark V. I just spent $340 on the Model 510 to 520 upgrade (now on sale). So I'm about $705 into this venture and I think I have a buyer for my old machine for $600.
So, for $105 I went from a 1982 Model 500 to a like new 520 (assuming my sale goes through). Considering that Shopsmith's price to go from a model 500 to a 520 and get the second bearing in the quill, is roughly $1,450, I have saved a bundle. I think you can make a similar saving, too - watch Craigslist and avoid both SS retail and eBay.
As for the Power Pro, my take is it gives you marginally more power and a bit higher speed. The speed will help you if you plan to route or shape with your Shopsmith but those are both jobs better suited to a dedicated machine - particularly the routing. I've got a dedicated router with just about every option you can imagine and that makes the PP/Mk7 a poor deal for me. I can't say I've ever bogged the saw down except when I was using a dull blade. I generally don't cut more than 2" stock (usually less) and I support it well, and cut straight. So far, no problems...knock on wood.
I say, watch Craigslist for a deal on a Model 510 or 520, sell your machine, and spend the difference on wood for projects.
Don
Sell or Upgrade
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