Important points for all of our server room builds for clients when it comes to racks are as follows.
Depth is hugely important due to the bigger 2 and 4 RU servers. The more space we have with depth the more room we have to work around them.
Temp sensor controlled fans is also hugely important as once you fill a rack with cables, switches and servers they can get a lot hotter than many would assume.
Cable Management. Now days we refuse to by any cabinet that does not allow for good vertical cable management as no one likes trying to troubleshoot a spaghetti monster.
Metal doesn't overly matter as its the racks usually become quite solid and rigid once you have placed a few four corner mountables in them.
You will also need to offer different height racks based on the fact that smaller companies will only need a 900mm high 22RU rack where as others may require a larger 42RU rack to meet their requirements.
I would not recommend carrying stock of these as they are always best to quote on a case by case basis to fit exactly the clients current requirements while allowing for a 5-10 year future proofing(Racks rarely get replaced unlike servers).
Hope this helps.