I have been doing as much DAT transferring lately as I have time. (these old tapes are not forgiving) I obtained a Fostex D5 from goodcooker which is a very solid workhorse.
You have the hardware, the DAT player. Make sure it is clean and works properly before you start.
Determine which outputs the DAT deck has, and decide from there which way to go. A deck like the D5 has digital out via SPDIF and optical. I despise optical cables, and most hardware you will be inputting to won;t have optical.
I have done both analog outs and digital SPDIF outs with the D5.
I am currently using a borrowed Tascam HDP1 with CF cards and an external battery (Maxoak 55,000mAh)
This is because of the bit perfect nature of it's digital input. Apparently, decks such as the Tascam DR-680, which I used when transferring analog out from the DAT deck, are not bit perfect.
That is my best advice for now. Good luck