What Do I Need to Get on Packet?
Well it depends :-)
The minimum for Irish packet (1200baud) is:
WARNING: The MixW TCP/IP over AX.25 adaptor may not work on Windows XP Service Pack 2, non-TCP/IP features of packet still work.
MixW isn't free. Flexnet32 and AWGPE are, but are MUCH harder to install. Since MixW is also good for PSK31, Contest Log book, regular Log book, CAT Control, Rotator Control, SSTV, Hellscriber, RTTY, CW (including DSP 100Hz for CW by ear), SSB antinoise DSP, MT63, Throb, HF Fax Receive, QPSK, FSK31 and Q15X25 it is good value at $45. You would pay much more for a 100Hz Collins filter or a DSP noise reduction speaker.
The Rig Interface is simple. If you can't home brew it, I'm sure someone would for you.
No transformers or capacitors or optoisolators are needed unless you get "mains hum". This is unlikely with modern PC.s Such an arrangement with minor variations works everythong from a VX1R (needs a capacitor to isolate Mic pin as it is also PTT) to an old Valve / Transistor Hybrid HF FT101ZD.
If the Rig has a "Data Port" with choice of 9600 and 1200, use the 9600 connection(s). They are audio in and out and NO resistor attenuator is needed. The 9600 connection will work with Flexnet32, AWGPE, MixW or Linux Sound card AX.25 on 1200 and 4800 baud too.
A few rigs have a TNC (Terminal Node Controller) built in. The Packet "Data" ports and modes of rigs are usually just Audio, not actual Computer Data as on the Serial data side of a TNC. These ports are designed for a "real" TNC.
Should I buy a real TNC (Packet Modem) such as a Baycom, MFJ or Paccomm?
Probabily not. Only a PDA or a GPS connecting direct to Handheld really needs a physical TNC. Most Modern PCs with MIXW will perform better and allow 4800, 9600, 19,200, HF 300 and HF 2500 (Q15X25). Most TNC for 1200 baud are inferior and only do VHF 1200 baud.
For Pactor II or III or any high speed packet mode you need an SSB Rig and specialist TNC/ Packet modem. Really for greater than 20K bps you need a dedicated Packet rig with built in "TNC". Such a radio can how ever be cheaper than a Pactor III "modem" or indeed a 2M FM rig plus HW TNC.
To get anything worthwhile from packet don't even consider a Dumb Terminal and conventional Paccomm / MFJ type TNC.
If you need a small, light wieght pocket sized TNC for mobile use the TNCX starts at $50 and can use a PP3 battery, Rig supply, a system 5V rail or a Car cigar socket. It is KISS compatible which means it will work with any GPS / PDA combination that uses KISS and also can do TCP/IP traffic. Any hardware physical TNC should be KISS compatible. But almost all PC and laptop users do not need a TNC below 19,200 Baud.
If you are creating a mountain top Digipeter in the past, a Paccomm Tiny-2 mk2 with separately obtained X-1J4 Net ROM (Includes TCP/IP routing) was the best choice. Now a Single Board Computer with Linux in a Flash memory simulating a hard drive is a better choice. It could control two radios via sound card(s) and allow a WiFi gateway to a Server off the mountain connected to Broadband for DXCluster as well as other Server based applications.