You have several main options to get a MT32-pi system:
1. Raspberry Pi (a 3 or 4) + a supported USB<->MIDI interface OR MIDI hat + a supported audio hat
OR
2. Raspberry Pi (a 3 or 4) + a supported USB<->MIDI interface OR MIDI hat, but just use the inbuilt 3.5mm audio jack for output
OR
3. WP32 McCake + Pi CM4 module
The 'normal' way is option (1) or option (2). This gives you an entirely standalone MT-32 / GM playback device. The recommendation is to use a dedicated hifi audio hat with the Pi, as the 3.5mm stereo jack does not have the best audio quality (it doesn't have a dedicated dac output), but these two options are functionally the same. For both of these options you need some way of getting MIDI data into to the Pi - that usually means a MIDI output from your PC (dedicated MIDI card, a USB<->MIDI dongle or a DB15/Joystick to MIDI adapter). There are various enclosure options available to make things look nicer (including PC drive-bay mounts, dedicated stand alone boxes, boxes which look like miniature MT32 cases etc - it's entirely up to you).
With option (3) you need a PC soundcard which has a waveblaster header. Then you attach the WP32 McCake and the Pi CM4 module. The emulated MT-32 / GM module is then directly attached to your soundcard and no additional hardware is needed. This is definitely one of the cleanest and simplest options to use, though it does mean you can't easily share the device if you have multiple PC's you want to use it with.
I have heard some people talking about using a Mister FPGA to run the MT32-Pi, but it seems like quite an expensive way to implement it, considering the relative cost of a Pi.
My collection database and technical wiki:
https://www.target-earth.net