SJ21s are relatively simple boats. They do not have a lot of systems. The hull is solid FRP with a cored deck. A survey is not going to teach you much about this boat that you would not be able to easily figure out on your own if you have prior sailing experience. The cost for the survey would be a waste of money in my opinion for a $3,500 boat that is pretty much what you see is what you get. Although $3,500 is steep for that boat. I would expect new rigging, sails and engine for that price.
Use SailingDog's inspection list and do the survey yourself.
Here is the SJ21 list. Jim Hubbard's San Juan 21 Sailing Page
They are fun to sail. I had mine for 4 years in Casco bay. Unfortunately there are not many in this end of the country so no active racing fleets.
BTW, I sold mine for $1,200 in 2002 and it was in very good condition.