Heartily recommend that you get a 2 track for doing live recording. 2 track at 15ips will give you the headroom that you need for recording live. I don't know what your budget is, but if you could swing a pro or pro-sumer deck like an Otari Mx-5050, Technics 1500, Tascam 32 or 52 (or similar) you'll be much happier with the results. Be sure that you set the bias and eq for the tape type that you will be using. Also, get some decent mics (AKG, Shure, A-T, etc) and a decent 2 channel mic pre-amp also (Joe Meek, PreSonus, etc.). A good analog compressor will come in handy too (DBX, Drawmer, etc.). OK, I know its getting expensive, but you will find that recording live is not so easy as the dynamic range is tough on mics, pre-amps, and tape decks. Without good mics to pic up the sound, a good mic pre-amp with plenty of headroom, a decent compressor to control the dynamics, and a tape deck that can handle the signal levels, you're not going to be very happy with the outcome. Having the right tools greatly improves your chances of getting a good result.
That being said, the 707 is a pretty good deck. I have one and (after I worked it over) it is very solid from 30 Hz to 20kHz (and beyond). The mice pre-amps in those generally need the transistors replaced (otherwise they are pretty noisy). I've never tried running mics directly into the 707, but I would imagine it would do pretty good job. I'd likely not want to attempt that though, as outboard mic pre's with compression between the mic pre's and the tape deck would give a much better result.