First, you should find out whether the Model 30 you are going to obtain is a 8086-based computer or a 80286-based computers. While both of these computers have a processor that can perform 16-bit data transfers, the "original" Model 30 only accepts 8-bit ISA cards (up to 3 of them), whereas the 286-based Model 30 can handle 16-bit cards.
Furthermore, the integrated video system of the classic Model 30 is an extended CGA-like system that supports the cheap and easy to implement VGA features: Software-configured character sets, 256-color graphics at 320x200 and monochrome graphics at 640x480, all of that on a VGA monitor. It does not support any EGA-like 16-color modes. You can add an 8-bit (Super-)VGA card, the onboard video will be automatically disabled then. The Model 30/286 on the other hand has the original IBM VGA chip on the mainboard. This is a quite slow, 8-bit VGA implementation, but it is the original VGA implementation.
The Model 30 (8086) has 128KB of RAM soldered to the main board, and usually comes with two 256KB SIMMs (industry standard SIMMs!), extending the conventional memory to the maximum standard amount of 640KB. The only memory expansion option you have on that computer is an 8-bit EMS board. On the other hand, the Model 30/286 was originally sold with 2 non-standard 256KB SIMMs, and can take up to 4 of this kind of non-standard SIMMs for up to 4MB of RAM.
Another recurring topic is the drives: The drives for the PS/2 line are proprietary, with power and data on the same cable. There are adapters to connect standard floppy drives, but mounting them in a way that they fit the case design is non-trivial. The integrated hard drive of the Model 30 8086 is IBM's own idea of creating an XT-attachment standard which is, to my knowledge, not compatible to anything else. The integrated hard drive of the Model 30/286 is an ESDI drive with proprietary connector, so getting spare parts is difficult, too. Adding your own hard drive controller (or hard card) is an option for both of them, so going XT-IDE with a CF card, or stanard IDE with a CF card (you still need an IDE BIOS with the 30/286, as there is no IDE support in the mainboard BIOS).
If you are going to play games, you likely want a sound card. Games that work nicely on the Model 30 family of computers generally will not support anything more fancy than a Soundblaster Pro, often just AdLib. Getting an OPL2 based card is obscenely expensive, as are all the original Creative cards of that era. I would advise against Sound Blaster 16 in this kind of computer, because you won't have any benefit from the 16-bit stuff, but you will lose SB Pro stereo compatibility. On the other hand, there are a lot of SB Pro compatible cards, often including WSS (Windows Sound System) support, for example OAK Mozart and OPTi 82c930-based cards, some of them having the Yamaha OPL3 chip or a perfect unlicensed copy. Those cards might be way cheaper and fit your needs.
If you want to get the machine hooked up to a network, you can get some card based on the WD8003 or an NE1000 for the 8086 variant (8-bits). For the 30/286, you can use one of the ubiquitious NE2000 clones (e.g. RTL8019), or a 3c509. You will have good driver support anyway. You can use NE2000 cards in 8-bit slots, but this requires special driver support, which is e.g. not included in the standard NE1000/NE2000 packet driver used as base for mTCP.
