There are really two flavours of SFF PC: those with a full-power GPU (typically for gaming or professional use) and those that don't. For the latter, it shouldn't be too hard to build something even passively cooled. You can go even smaller. I know less about this space because it hasn't been my focus however.
Gaming SFF PCs are my jam and here the Louqe Ghost S1 holds up extremely well:
1. It takes almost all double-slot cards. Getting something a little shorter can help in terms of getting the card in and out and for cable management but there's really no need to shop for a specific shorter model. The more than two slot cards (eg RTX 3090, RX6900XT) won't fit but say a 3080 (assuming you can even get one) will be just fine;
2. My personal preference for PSU is something like the Corsair SF750. It's a 750W highly-rated SFX form factor PSU. It works well, can power a 3080 system and seems to be relatively quiet;
3. The Ghost with a top hat can then mount something like an NZXT Kraken X530 AIO to cool your PC. This is going to cut down a lot of your noise; and
4. With a separate top hat on the bottom of the case you mount two quality Noctua 120mm fans in exhaust (meaning it pulls air from the case).
The net effect of all this is that air is drawn in from the sides and expelled out from the top and bottom. It is also a negative pressure set up, meaning there is more force for exhaust than intake.
Positive pressure would be nice for keeping dust out of your case but negative pressure here is better for airflow and thus noise (by operating at lower RPM).
The above will have almost full case power and airflow and be relatively quiet. It won't be as good as you can get with a full or mid tower case but it's pretty darn close.
Gaming SFF PCs are my jam and here the Louqe Ghost S1 holds up extremely well:
1. It takes almost all double-slot cards. Getting something a little shorter can help in terms of getting the card in and out and for cable management but there's really no need to shop for a specific shorter model. The more than two slot cards (eg RTX 3090, RX6900XT) won't fit but say a 3080 (assuming you can even get one) will be just fine;
2. My personal preference for PSU is something like the Corsair SF750. It's a 750W highly-rated SFX form factor PSU. It works well, can power a 3080 system and seems to be relatively quiet;
3. The Ghost with a top hat can then mount something like an NZXT Kraken X530 AIO to cool your PC. This is going to cut down a lot of your noise; and
4. With a separate top hat on the bottom of the case you mount two quality Noctua 120mm fans in exhaust (meaning it pulls air from the case).
The net effect of all this is that air is drawn in from the sides and expelled out from the top and bottom. It is also a negative pressure set up, meaning there is more force for exhaust than intake.
Positive pressure would be nice for keeping dust out of your case but negative pressure here is better for airflow and thus noise (by operating at lower RPM).
The above will have almost full case power and airflow and be relatively quiet. It won't be as good as you can get with a full or mid tower case but it's pretty darn close.