The TiVo has around 2 weeks of guide data, I find about the nearest week reasonably accurate, the week out a bit sparse.
You don't need to hack the TiVo itself.
You don't need the TiVo adapter, but if you want to go wireless, it is the recommended choice. If you have a Series 2 DT, it has a built in Ethernet adapter. Older Series 2s need a separate ethernet adapter, or can use the TiVo wireless adapter. Series 2s can use some other wireless adapters, but it isn't worth the hassle looking for them, and having to use WEP..