Stick 4x 1MB SIMMs in there for maximum RAM. It doesn’t seem to care about DRAM speed or parity, so any will do!
It’s very common to find machines that have had the right side of the case mutilated to fit a standard PC floppy drive or a Gotek.
If you fancy having an internal Gotek drive, I can recommend one of these:
https://centuriontech.eu/product/goex4st/ — it’s basically a custom Gotek that fits perfectly inside the ST with no case modifications.
Personally, I have one of those, plus a Centuriontech replacement internal PSU. The PSU has an auxiliary 5V output, which I use to power an external UltraSatan Mini which is another SD-to-ACSI board. (ACSI is Atari’s SCSI equivalent).
You’ll need hard disk driver software to make an SD card usable in TOS/GEM. I have only used ppera’s driver, but I like it because it creates PC-compatible FAT12 partitions, so I can pop the SD card in my PC to copy stuff onto it.
ppera also maintains a massive library of game downloads, modded for hard drive use and for compatibility with STE, Mega STE, Falcon030 and TT.
The extended joystick ports on the left are next to useless. Very few games support them, and the only controller for those ports is the Jaguar’s JagPad, which you’ll either love or hate. I love it, but I admit I’m odd.
Get an ST2VGA adapter (also from Centurion). ST-HIGH resolution has VGA-compatible timings, but is monochrome only. LOW and MED resolutions require an upscaler of some sort to turn their signals into something all VGA monitors can handle (I use an OSSC). A handful of VGA displays can handle the slow 15KHz horizontal refresh of these modes, but only a few.
I have a soft spot for the ST — it’s the machine I grew up with. Have one of each series — an FM, an E, a MegaSTE and a Falcon030. Love them!