I had a '77 E27 for over 20 years. They are a Skookum built boat and not a lot of inherent problems with them. The chain plates have been mentioned and the other thing is to look at the mast just aft of the spreader attachment point. There was some problems with the mast 'dimpling' in that spot from the spreader pressure. Other than that it's mostly just maintenance issues specific to your boat and what the PO's did (or didn't) do with it.
As far as upgrades, the best things I did were:
1. Added Roller Furling
2. Added a dodger
3. Took out the A4 (was shot) and put in a diesel.
4. Autopilot (I had a wheel model).
5. Cabin heater ( I added both a Red Dot off the engine coolant and a kerosene heater).
I will warn you after I added the roller furling on the jib I rarely used the main. I'm a cruiser not a racer, and putting up the main vs a 110 genoa only got me another half knot. Not worth the effort for the gain.
I'm in the Pacific NW. Having a dodger to get out of the weather is a huge improvement in livability on the boat. Both from rain, wind, and even the sun when it finally decides to come out. With a wheelpilot added with a remote control, I made a small seat that fit over the first hachboard in the companionway. I could sit on it out of the weather with my lower body in the warm cabin and beat or motor into the nastiest weather in comfort. Also if you are at anchor, you can have the hatch open regardless of the weather. The cabin of an E27 gets a bit claustrophobic after 2 days in a downpour. Just having a covered front porch that you could leave the door open on made a big difference.
The diesel was a great addition for me. I would not suggest replacing an A4 that was in good working order but mine was shot. It was a big expense to do but over the next 20 years I got every penny out of it. Not just the reliability and safety of a diesel, but it increased your motoring range by a factor of 4. The A4 burned about 1 gallon / hr. The Perkins M20 I put in was about 1 qt. / hr.
Depending on your location, this list may or may not be what you need most. But you've got a great boat that will serve you well if you take care of her.
Good luck!
Kevin Wright
E35 Hydro Therapy