1. Yes, Computech is legit. I bought my Logitech Z623 speakers from them. They shipped quickly and in excellent packaging.But both are out of stock in both sites.
Here it's available but I never bought from this site. Is it trustable? Anybody from TE bought from these guys? The above Deepcool PM750D is priced at 6.2k here. Probably I should have bought from these fellows in the 1st place before buying CM 750w V3 PSU. I was in a hurry and I made a mistake.
2. Is your current PSU giving you any issues, or are you just worried because of the tier list? If it's the latter, and the PSU is working fine otherwise, don't stress. Just because it's tier C doesn't mean it will go kaput. I've had plenty of shit PSUs that lasted years under heavy load.