Hello Bell47 and welcome to the forum!
I moved this topic, because it fits in better here.
I have my transducer cable extended to 50m (150 foot) and have good results after some initial problems. Check out the posts here in the DIY section and you'll get lots of information. Rickard, Abra and Kron have posted their experiences here in the last couple of weeks.
I used a CAT 6 patch cable and spliced it into the transducer cable. It's a bit of a challenge to get the wires soldered together and insulated, but it's doable with the right tools and some patience.
As for the unit you want to buy: Get the biggest screen possible. I wouldn't buy anything smaller than a 900 model. The 998, 1197 and 1198 have a video out port if you want to get a screen hooked up. I'm not sure if a laptop will work as a bigger screen. It must have a RCA input port to get the unit hooked up.
WayneP posted about his 1197 hooked up to a flat screen TV recently:
http://forums.sideimagingsoft.com/index.php?topic=1930.msg10508;topicseen#newI have my unit completely isolated from anything of the boat's electrical system to avoid interfences and I highly recommend such a set up.
We have a poll here in the tow fish section about transducers with extended cable length.
http://forums.sideimagingsoft.com/index.php?topic=1774.0Please vote there as well, because we want to get as much people together to show Humminbird that there is some interest in such transducers.
I'm planning to build a 100m transducer to get good images from greater depths of our lake here. But if I can buy one directly from Humminbird, I would rather go for this than fiddling around with solder iron and tiny wires.
Best regards / Harry