Don't think you'll be looking at sli, that's for gtx1070 and up and thats not going to happen on a $500 budget. The rarity of current mid price amd cards due to crypto mining is going to shoot you down too. Which still leaves the Quadro cards as a viable solution, but that'll depend on your monitors. If you are using 1080p, I'd say a single Quadro p2000 could handle the whole thing easily, as it's got enough juice to power 4x 5k monitors separately via DP. Having 6x monitor 's would mean the use of MST, either daisy-chaining DP in/out capable monitors (upto 3x per DP) or via MST hub which basically creates an extended window across multiple monitors. Using 3x hubs on DP would get you your 6 monitors. Basically 3x sets of extended displays, or use 2x hubs across 3x displays, lower 3 and upper 3 etc.
The issue with sli/xfire is (to my knowledge) that the master gpu has all sli connections, the secondary cards are just additions, they'll only use their ports when used independently of the master, which kinda kills the point of the sli in the first place.
Quadro p2000 is $426. An MST DP is @$120. It's a little over budget, you could drop down to a p1000, but it's a much lesser card. With 4 DP per card, you could power 3x independently and 3x on a single DP extended.
If using higher than 1080p, looks like you'd need 2x p1000, not used sli, but independent, which would power upto 8 independent monitors, but those are 2Gb cards vrs the 5Gb of a single p2000. The p1000 is $330. Could even go further down the chain, a P600 is $180, so doable under budget, and will handle Photoshop etc without issue. Any 3d or Cad work might be a little pinched though.
The bonus with the Quadro's is the warranty. It's entirely possible that if a card develops issues, you'll either be on the phone with PNY and/or nvidia tech within minutes to solve the issue, or get a replacement within hours. A normal retail gtx could take a couple weeks for RMA.