It depends on how much you want to spend. There are lots of decent transducers like the Airmar TM150M, TM165HW, TM185M OR TM185HW. Personally, at those depths, any of them will work well. The TM165HW would be my preference since its 600W. The TM185 series are much larger and expensive, but its 1KW. Even the general purpose Active Imaging 3-in-1 transducer should be fine at your depth range.
If you worry about reading bottom at planing speeds, you just need to learn how to adjust the transducer properly. It will never outperform a thru hull transducer, but you can get decent results. In my experience, you will get more noise with the medium and low frequency transducers as compared to the high frequency transducers. You get more bubble noise and so on. The high frequency transducer does not penetrate as deep, but at 180 feet or so, it should be no problem. The transom mounted transducer also tends to throw out a rooster tail. If its close to your engines, make sure the water doesn't go into areas like the shaft and cause it to corrode and bind. Removing a lower end with the shaft welded to the engine is not fun.
Never get a "budget priced" anything when it comes to a thru hull transducer. In general, get the best you can afford. You're a lot more likely to change your electronics than to replace the transducer. The Airmar B175 series of transducers are great transducers. They also work with a range of different fish finders with the right adapters. The thru hull transducer are definitely much better at planing speeds as compared to transom mounted versions. Some boats require more preparation to use them like a foam cored boats. Other boats like the Triumph boats are made of plastic so it won't work either. The flexing of the boat can cause the sealant to break and result in water intrusion.
