My first PC had an ST-225, and I'd like to confirm that it is indeed MFM. (I miss the gronky startup sounds from that thing.)

If you want to modernize the storage for that old PC, then Lo-tech is probably a good choice. They've got a wonderful all-in-one card that includes a mounting bracket, fits into an 8-bit ISA slot, and accepts a CF card directly: https://texelec.com/product/lo-tech-xt-cf-lite-rev-2/

File transfer can be done a number of different ways, including serial ports (USB to RS-232 adapters generally work fine), floppies (USB 3.5" floppy drives are awesome and cheap), and: Networking.

Around the turn of the century, I briefly had an already-old XT machine that had no hard drive, but it did have 10BASE2 Ethernet and Hercules-compatible graphics built into the motherboard.

It was a fun little adventure to get it online with my then-modern LAN and I did a number of unlikely things with it (including a graphical web browser), but the most practical thing it did was connect to a Samba share on my Linux box. It worked great. Old 8-bit network cards are not particularly expensive.

It all depends on how far down the rabbit hole you want to go. :)