The top three things to consider when buying an boat are; Condition, Condition, and Condition. I suggest that you ignore all the electronic doo-dads and only look for a well maintained vessel.
The Boat Buying thread, started by SailingDog years ago is a great place to start. To summarize it; a sailboat consists of three, or four, main systems;
- Deck & Hull
- Rigging (Standing & Running) and Appendages (Mast & Boom)
- Engine and Drive Line (optional)
Look for a sound deck and hull. There should be no spots where either of them flex from any pressure that you can apply. Walk all over the deck, push on the hull, look at the boat on stands; there should be no flex anywhere. Go a step further, and spend $300 on an Electrophysics CT-33 Moisture Meter. This will help you determine if there are any hidden surprises.
Look at the standing rigging. Take a cotton rag to the stays and shrouds and rub it up and down. If the rag sticks, then you have likely found a broken strand, or "hook" in the rigging. Any hooks in the rigging mean it is time to replace. Look VERY closely at the chainplates and turnbuckles. There should be no cracks at all. Rust tends to form in cracks, called crevice corrosion, and will be visible. Also look over the running rigging (lines, shackles, blocks cleats, etc.). The line should not be frayed anywhere, shackles and blocks should be inspected just like the chainplates.
Look at the mast & boom. There should be no cracks or obvious patches. Because they can weaken the appendage, there should also not be too many unused holes, from old stuff that was removed.
Raise the sails. They should fit the rig, and not be too short, or long. They should not have obvious patches. Look closely at the stitching of the sails. The stitches should not be frayed, and should be intact.
Because the boat may, or may not have an engine, I'll leave it off this list....