Some more or less random thoughts:
I started sailing in the summer 2003. I have used a mooring since 2004 and I love the mooring! Easy to get the boat on and off the mooring, cheap, and safe. I row out to mine in a cheap dingy. Very simple.
Diesel Engines and Maintenance:
I have owned my current boat since 2006. It has a Universal Diesel M25 that had about 3600 hours on it when I bought it. In the five years I have owned the boat I have put 250 hours on it. The only 'engine' work I have done is oil and filter changes, fuel filter changes, and an electric fuel pump replacement, and a new exhaust mixing elbow. Diesel engines are pretty simple and very reliable.
I do need to do some electrical re-wiring for my engine, but you must expect to do some work on 25 year old boats.
Getting Involved in the Sailing Community
Go Racing! Seriously. Find a local yacht club or sailing association or something like that and ask if anyone needs crew. I assure you that many captains are looking for crew. There is no better way to learn about boats, sail trim, seamanship, tactics, gear, etc. than to get on the water on some one elses boat and do it for free! Maybe you like it and want to do more, or maybe it's not your thing, but you'll get to climb all over different boats and see which things you like and which you don't.
In the 70's features like hot and cold pressure water and AC electrical systems weren't so popular. By the mid 80's they were. And not just on high end boats like Sabre. Most mid 80's Newports (27, 28, 30) came with teak and holly cockpit sole, leaded glass cabinet doors, pressure hot and cold water in the galley and head, shower in the head, and AC electrical systems.
My second boat was a Newport 28. I originally wanted a Catalina 30, but good ones were out of my price range. I was at a boat yard looking at a S2 9.2A (which was junk) when the broker pointed out a Newport 28. Hmm, this is pretty nice. Self tailing winches, all lines led aft, good set of sails including assym spinnaker in sock, autopilot, Universal Diesel M18 in good condition, wheel steering with Autopilot, hot water, berths for my family of 5, shower in the head, etc. Long story short, I bought the boat and enjoyed for 3 years.
For me, having the 'luxury' features made my family comfortable and more willing to spend nights aboard.
For $20K you will have no problem finding a very nice mid 80's boat with all the features. For that amount of $, expect the boat to be in 'sail away' condition with good sails and gear.
Here is a simple list of boats to whet your appetite:
1980 (Sail) Boats For Sale