Hi and welcome to the server builds community.
There are a lot of different topics going on in this thread some of which could easily be their own stand alone discussions.
TrueNAS Core vs Scale
The main difference between the two boils down the the OS used. Core is built on FreeBSD and Scale is built on Linux specifically Debian Linux.
FreeBSD is very lightweight so you get great performance even on old hardware and ZFS support is very mature and rock solid stable. However the installed base and community is much smaller than Linux and as a result there are far less options for compatible software, new hardware takes much longer to get driver support, and there are a lot fewer resources out there for learning and support.
With Linux you get much better compatibility in terms of both software and hardware and there is a huge community and tons of resources out there to support it. ZFS support is a little less mature but it has come a long way.
I would probably only recommend Core if you only wanted to run a pure NAS without other services and you were using very old hardware. For the use case and hardware you mentioned, if you are certain you want to use TrueNAS, you would want to use Scale. However that brings us to the next topic.
For the use case you mentioned I would highly recommend Unraid. Unraid is designed from the ground up to be a very user friendly NAS / Media Server. It is easy to get set up, provides a very user friendly interface for running docker containers and VMs, and the JBOD array setup is easier to use and expand than a ZFS RAID as well as being much safer in terms of not destroying your data by way of user error.
ZFS is an amazing file system. It is loaded with great enterprise level features and you can spin up 20 disks and completely saturate a 10 Gbps or faster network connection 24/7. However in reality that is never going to happen in a home server environment. A single disk will easily saturate a 1 Gbps link and you probably won’t even do that most of the time. The downside of ZFS is that with all those great features comes a very steep learning curve and the one thing ZFS does make very easy is to permanently eradicate all of your data with a single miss-typed command.
The only situation I would recommend TrueNAS over Unraid (for home use) would be if your main goal is to learn to use ZFS, however at the same time I would never recommend learning to use ZFS with data you care about or want to keep. Do your learning in some sort of sandbox environment with expendable data.
You can still use ZFS with Unraid although not quite the same way you would with TrueNAS. It works quite well for cache pools, and you can even use it to good effect in your main array, although it will be individual disks rather than zpools.
The Youtube user SpaceInvaderOne makes lots of great explainer videos on Unraid including the use of ZFS. If you do go with Unraid you should check them out: https://www.youtube.com/watch?v=rEAfX75nReg
First Steps after building your Server
You might need to go into the BIOS to reset settings to default or select a boot disk, but really the first thing you will want to do is get into the IPMI. There you can update your BIOS and BMC firmware and remotely control the system.
With a used motherboard, if it has not been reset to the defaults by the seller, that can be a bit tricky. The default IPMI password for your SuperMicro motherboard is ADMIN/ADMIN. However if there is a non default password you will need to boot into an OS (Probably Linux or FreeDOS) to run SuperMicro’s IPMI Tools to reset the password. Here is a good guide on that: Reset Supermicro IPMI Password to Default - Physical Access - STH
Hosting services publicly accessible on the internet
Pretty much any OS you choose will easily facilitate spinning VMs to host servers that you can use for app and web development. I highly recommend doing this on your local network. There is technically nothing stopping you from making these servers available to the internet at large, but as others have pointed out that is a whole can of worms that I would strongly recommend against opening.
There are many concerns with connecting servers on your home network to the internet. A typical residential internet connection is very low bandwidth, especially in terms of upload which is what matters the most for a webserver. You would be very vulnerable to ddos attacks which would effectively kill your internet connection to your home and might incur overage costs or the possibility of disconnection by your ISP. If you are not absolutely on top of security at all times you will also be susceptible having your server hacked by bots on the internet and any attacker would then have access to your entire home network.
A home server is great for doing development, but when you are ready to host services on the internet you would be much better off renting VMs from a hosting provider in an actual data center which will have a much faster and more secure network and things like built-in back up services and close to 100% up time.
Hope that helped answer some of your questions and whatever route you decide to take enjoy your new server!