I would just recommend staying far, far away from it especially if you're building new. Even if it were free, the best thing you could do would be to sell it for what you can on Ebay, then go buy a TruTrak, Trio, Dynon, GRT or Garmin autopilot for a good price. Then you'll have a modern autopilot that doesn't weight a ton, works well in experimentals, and have support from the companies. Also interfacing them with modern equipment and getting them adjusted properly can be a challenge to say the least. We're in the middle of pulling out a S-Tec system from a custome airplane right now (and have done so in the past).
They do work....but no reason anymore to put one in when all of the competing products are only about $1500-$2500 for an entirely autopilot, brand new from good companies. S-Tec is owned by Cobham (remember Chelton)?!
Just my 2 cents as usual.
Cheers,
Stein