I buy a lot of "legacy" videotape machines for use in my business and I wouldn't get them serviced unless a machine displays a very distinct need for it. Hook it up, put in a test tape (i.e.: one whose content you're very familiar) and then get it working within your system. If you can, record onto a new tape a few minutes of bars and tone (Blackmagic supply these in their Desktop Video setup package) and then compare playback with source (you might need to clean the heads etc first).
If playback is slightly noisy or you have white flashes on the playback picture, or if the meter that displays the video playback strength ("RF Level") displays a low level, it could be that the heads are worn and need replacing. This is a very expensive job and isn't worth the bother or expense these days. However, if the playback is good then you're good to go.
One word of advice - a few years back I had exactly the same configuration as your proposed set-up - I used a TV-One 1T TBC feeding a Shuttle Thunderbolt into MacOSX. However, the TV-One 1T isn't really a timebase corrector as your recording will drop out when there dropped frames in the VHS playback. The version that has a Gunlock input is slightly better and more tolerant because the device is referencing an external sync source to which it can then lock, but it still isn't perfect. However, if the recordings are continuous without any breaks then it should be OK.