A few things you should look out for when choosing somebody to transfer your tapes.
1. Ensure that they don't use a standalone DVD recorder and that they don't encode straight to MPEG-2
2. Put a maximum of 1 hour on a DVD5 or 2 hours on a DVD9 (dual layer)
3. Ensure that the head switching noise (located at the bottom of the picture) is masked
4. Check whether they use a TBC when digitizing your tapes
5. Check to see if they can process your footage (eg. light DNR, de-noising, chroma correction)
All of these are really great suggestions...and I'd add that each one, in its own right, is almost equally important to making sure you get a good end product. It should be fairly intuitive why the first three are important. Number 4 is huge and I wouldn't attempt a VHS conversion project without a TBC because you'll almost 100% surely have tracking issues that won't transfer properly if you don't use a TBC. This means that some (perhaps alot) of your digital transfers will end up with annoying tracking glitches if you don't use a TBC. Number 5 is also quite important because in my experience old VHS recordings/tapes aren't uniformly well preserved. Seems like every tape I convert needs or benefits from some kind of digital enhancement in post (in my experience, mostly color correction).
FWIW, I have a TBC I may be willing to sell, but I'd probably want more than most people would be willing to pay. I paid $300 for it some years ago, but wouldn't sell it for less than $200 since I still get some good use from it.
EDIT TO ADD: It's been a long time since I've touted the benefits of a TBC box, but I'll never forget the thrill I got at the magic it worked on some VHS tapes I had once considered unplayable because I couldn't get them to play in a replacement VHS machine. Turns out that the tracking was just incompatible between the machine the tape was originally recorded on and the newer machine that I tried to play through. The TBC did whatever it does to digitally correct the tracking to restore the tape for me. I was both floored and thrilled to have some old family video restored to new...after thinking they were lost. Those 2 or 3 tapes alone were worth the price I paid for the TBC.