. Erase sda1 and sda2, create new sda1 as ext4 primary with mountpoint at /
. sda3 is kept as originally partitioned and formatted with ntfs
This is correct if you will stay with MBR partitioning. Nothing wrong with that in most cases. UEFI installations require an additional, small, EFI partition made as FAT32, and those default to the newer GPT partitioning method, but you do not need to do that.
GPT can be useful in some cases, since MBR limits you to a maximum of four Primary or Extended partitions; to exceed four, you have to add Logical partitions underneath an Extended partition. GPT does not have such a limit; everything's Primary, no Extended, no Logical.
Do I rename sda3 to sda2 or does Ubuntu also create a loader partition sda2?
Don't worry about the numbering of the partitions, it is all handled automagically.
I think that I have enough ram so I thought that I would forgo the swap partition.
Installation of modern versions of Linux (since Fall of 2017) automagically create a swap file instead of a swap partition. You can manage a swap file far easier than a swap partition, even on the fly without a reboot, and adjust the swappiness to reduce how often swap get touched.
Let the installer make the swap file for you and don't try to run without, as a system crash could ensue if you overrun RAM without a swap file. Swap death is no fun.
Should I someday change my "D: drive" to something besides ntfs?
Eventually, copy the contents of the NTFS partition to a native Linux partition (default is ext4 but there are others), erase, and reuse the space of the NTFS partition, as Linux does not have the best set of tools to deal with NTFS filesystem problems.
However, you left out the most important step: Backup.
Make a backup of your data, then verify the backup copy matches the original (checksums are great for this). Then, make another backup to a different physical device (or into the cloud) and verify that.
Why? Changing drive partitions, deleting, creating, and formatting, is the easiest way to lose gigabytes of data with one mistake, brownout, blackout, or power spike.