(more of a rant)
We're a Dell and HPE partner, and I have to say the buying process has been pushing me more toward Dell. This is geared toward the manufacturer's site, rather than resellers.
Dell's process: Find your starting point, customize what you need, and send the URL to your partner. The URL has all of the config in it, which is convenient. I also value the hard requirements on the customization, such as not being able to add dual SD micro SD cards in the build, without choosing a combo reader (wouldn't want to have to go back and individually order a part you forgot when the system arrived).
HPE's process: You can do one of two things, assuming you need to actually customize it (99% of the time, I do), rather than just use the pre-built offerings on the site which are normally very incomplete for the needs. You can either download a PDF that has all the parts for that server. This can be tough to find, so you may as well use a search engine such as Google to do a Quick Sheet search for the model you're thinking. Once you find the link you're looking for, and there is a PDF with all the available part numbers, you can start collecting (don't miss anything!). Collect all your part numbers, send it off to your partner, and begin going back and forth on an email string about questions from the partner about anything additional/conflicts/etc. Your second option is reach out with "I need this much memory, this amount of drives, a controller that has this much cache, these kinds of drives, these NICs", etc. Worth noting... HP (HP Inc) is a reseller for their other half, HP Enterprise.
I like to procure different manufactures throughout the year to see if things have become more convenient with one, better navigation, different useful features added/etc. But holy shit has HPE become more difficult to procure.
SAN appliances are another nightmare. Call for anything and everything. Do I have an hour to talk on the phone about something I could spec out in 5 minutes? No.
Edit: Please feel free to offer any advice on spec'ing servers/storage out through both vendors if I am missing something (spoke with our procurement and HP yesterday, so I don't believe I'm missing anything). Again, I'm not comparing HPE/Dell's sites to going through a reseller/partner since we are a partner.