Speed is not an issue with the correct transom mount transducer set up properly. Here is a link to a thread showing an Airmar TM260 transom mounted transducer reading bottom at 50+ m.p.h. Scroll down to the video in post #10.
http://www.thehulltruth.com/marine-e...vid-added.html

Properly set up a transom mount transducer will work great. Now, your trailer clearance problem is another issue.

You can buy the factory Lowrance shoot thru transducer for around $60. You will have to make sure it is epoxied parallel to the water. If you don't have a spot in your bilge that is flat you may have to build a small wedge out of fiberglass and glass it to the hull so that your transducer is shooting straight down and not off on an angle.

http://www.cabelas.com/product/Lowra...h-All+Products

I would consider the Lowrance HDS-5. It will do everything you want it to at a decent price.

Here yu go. $629 plus you get a $200 factory rebate. Then buy the $60 ducer and you have $489 in a brand new HDS and shoot thru ducer.

http://www.cabelas.com/product/Lowra...h-All+Products