First, thanks for your answer, it helps a lot. Now about your advice in detail:
You need to upgrade all the external PSCs first, followed by vCenters.
Ok, good to know. I will get some more knowledge about a PSC.
What you actually have is a form of NIC teaming and trunking. Route based on port ID allows all VMs that share that port group to be balanced between those physical NICs in that VM01 may be put on vmnic1 and VM02 may be put on vmnic2. There's nothing else you need to do here. It also allows for failover so if vmnic1 fails, whatever VM vNICs utilize that vmnic will be moved to other available vmnics.
Ok, sounds good. Then I will not touch this,
I know you said this was a dev environment, but it's still "production" in that it is a business-reliant environment, correct?
Well, yes it is.
See my introductory paragraph for this one. I would strongly recommend against this one until you have hardware that is officially supported on the HCL.
Hm... first, I highly doubt that I will get certified hardware in a reasonable time. Most likely, the next point in time where I can raise this question to my superiors is when the current hardware is outdated, probably in a year or two. But, from what I have discovered so far, may only choice is to switch out the whole server, right? As I havent found a list of supported Mainboards, RAM and HDDs in the HCL? Or am I looking the wrong way?
I still would like to upgrade now to have some control over it and to not be forced to do an update when it gets urgent. At least I want to get rid of the VCSA 6.0, as the webinterface is awefull in that particular version.