I installed a stainless telescoping ladder on my 21 ft Hunter daysailer. The ladder was only $125 from West Marine and they have a variety to fit to your particular transom shape, including ones with stand-offs for the flat stern you have. Because it was offset on the stern (due to the rudder), I had to do some shimming with Starboard to have it vertical when it was in the water.
We have those Daysailors at our community sailing school - nice little boats. Their main feature is that they are remarkably light (600 lbs) so have light decking. I suggest buying some serious SS backing plates from a local metal shop to avoid pulling the attachment points out of the deck. Due to the leverage, the force you will get on the attachments when a 200 lb person steps on the end of that ladder will be on the order of 1,000 lbs! If you get a ladder with transom standoffs, you may want to reinforce those areas as well so you don't shove the transom in when boarding.