IMO 2- 150w DEs with the PCs will be fine for a regular 90, only 24' high, right ? however if you use 20 k might limit you on some very light hungry species. Higher the K the lower the par. Too blue for my taste but Im not tasting them.
I think thats a fair price for DEs, mogals would be cheaper. I dont think they make DE in 175 but still should be able to find a 175 in 20K if thats what you want. IMO go with the 150 DEs.