Recommendations for PC -- for modest system --

DIY? -- Can I approach equal or better quality - for less if I DIY with the likes of with neo or dh cable or other with WG or other connectors... Is it worth the effort to DIY at budget I have set?

No. Not even close. I know one guy with 30 years building DIY cables trying all the hot schemes who every time he showed me one I had something in my used cable drawer that we BOTH agreed sounded better and cost less than he spent on parts. Waste. Of. Time.

Synergistic Research - considering - but I see none in the <$500 range...

A used SR Master Coupler is under $400 and a darn fine power cord. A used Blue can usually be found for $500 or less. Synergistic is so consistent quality wise you can be safe going with any model in your price range, and generally the newer the better. If you find one with Active Shielding those are a great choice because they'll sound good stock and can be upgraded with better caps and diodes in the wall wart by Michael Stallone for a huge increase in performance for something like $50-75. 

I just built a PC with DH Labs Red Wave bulk cable and Furutech FI-28 (R)/FI-28M (R) rhodium connectors and it is really good. Five ft cable and Furutech connecters for about $400 in parts. You can use generic rhodium AC connectors from Parts Connexion which cost much, much less than the Furutechs. The Red Wave was $800, and is now available for $560 assembled from factory but comes with less resolving Wattgate connectors than listed above, so you can really save some money building yourself.

Another home brew cord I am high on is the Nanotec bulk cable which you can purchase from MCRU in the UK for about the same price as the Red Wave. Use either the Furutech or Parts Connection rhodium connectors and for $300 to $400 you get an a $800 cable.
The Nanotec is a little smoother than the Red Wave, but perhaps a bit less revealing. Both are excellent.

Other good cords I have tried:

Black Shadow with Rhodium connectors often available on Audiogon for under $200 is a real steal.

The IsoTek EVO3 Premier AC power cable is another good performing cable for the money, but pretty lit up if that is your cup if tea.

I am using a Shunyata Venom HC power cable on my Shuyata power strip that supplies everything but my amp and sub and it is pretty good but had an ungodly long breakin period. Its available in 15A and 20A versions fir $295.

I have not listened to AQ’s newer cables, but I hear good things about them. I am running some of their older PCs on my sub and in an office system. If you can get that kind of pricing you should look seriously. You could score a Blizzard or Thunder PC for under $400.

I would not hesitate to call the good folks at the cable company and get their opinions.

I heartily recommend Audio Envy.

Cullen and Audio Envy would be my budget choices, and I believe you can return them if they don’t provide a significant upgrade to stock.  Best of luck. 

Unless you're having a specific problem, I'd save your money and use stock PCs in that system. Your money is best used to upgrade your existing components, imo.