I found a used Iwaki IWAKI MD-70RLZT that I could purchase for about $150. Rated for 1500gph, is that enough turnover for a 350g tank? Turnover would be about 4x per hour. My most recent tank idea is a 6x4x2 so it will have a much wider footprint than the 210g tank I was previously looking at.
Honestly no that is not sufficient. Rays produce a substantial bioload and need over filtration. Especially with lower water volume for immediate dilution. For a tank that size I would bbe running 3,500 GPH but no less than 2,500 GPH. A quality pump like a Reeflo Dart would be perfect. If you prefer submersible, I also like Laguna brand pumps (2900 or 4280 would be great). Some folks stand behind Jaebao pumps as well but I’ve not had any experience with those so can’t speak to them.
I used metal (2” steel) stands but there are people by the bucket full who use DIY or professional wood stands. The advantage to me is peace of mind and more access underneath the stand. Sumps for rays get big, quick.