When searching for the right trailer launched sailboat, consider if the boat will need a trailer tongue extension to launch. Boats like the low slung in the trailer SanJuan21 pictured above can be launched without an extender (provided the launch ramp has enough slope). This makes it much easier to launch if you are dry sailing the boat (one that is stored in a land stall rather than a marina slip). Most of the Macgregor model cruiser boats are like the SJ21. Many other hull shapes where the keel retracts completely inside the hull (Siren 17) will sit lower on the trailer than a boat with an external swing keel trunk or a fixed keel sailboat. These types will sit much higher on the trailer and most likely need an extension on the trailer to launch the boat into deep enough water to float the boat off the trailer.
How the boat is supported on the trailer is important in that there should be no hard pressure points that cause indentations of the hull. You want to spread the load over springy bunks and supports. When trailering you must make sure the boat is properly tied down so that it doesn't bounce off the supports when on the road. The bow eye should also be tied straight down to the trailer as well as the recovery winch. There are a lot of really good articles on SJ 21 websites regarding proper use of the winch, keel supports, proper technique when raising or lowering the keel, etc. etc.. Really important stuff! Get something wrong and you have a big project fixing the swing keel.
The SJ21 is a really good sailboat for a lot of reasons. It is however more race oriented so generally things are a bit lightly built and it has a small, sparse cabin interior. There is a lot of good information on the SJ21 websites and some sailing club websites. There is a very active fleet at Glenmore Sailing Club in Calgary.
Ranger 20
The Ranger 20, although it is an older design has a reputation for being solidly built.
It does have a stub keel so requires a bit more depth to launch than the SJ21. Rangers were built in the Seattle area and some in Utah. There are around 600 of this model built.