Looks like a nice boat but the asking price is exceptionally high. Sounds like the owner is trying to recoup the costs of the re-power. I bought my A35 three years ago (1962 Hull# 54 - Auriga) and paid $7500. They were originally asking $19000. It was in about the same shape as the boat you're looking at. It had been well cared for but as in your case, nothing had been upgraded (except the engine) for many many years. It had a relatively new Universal M25XPB which is the exact same engine as the Beta 28. Both are marinized versions of the Kubota D1005. Strangely enough, Beta rates the output at 28hp while Universal/Westerbeke rates it at 26hp and Kubota rates it at 23.5hp. Not sure how each came up with different ratings but I'll trust Kubota. This M25XPB is just about right for this boat. I've had to fight some fairly strong and its close to being a bit short on power. I think an A35 will be a bit underpowered with a Beta 20 but maybe they worked some magic with prop sizing.
Deck delamination repair is a big project. Don't let anyone trivialize the efforts involved. My deck was about 90% saturated and it took me about two months full time to complete the re-core. It's a very weather dependent project. This would probably be the biggest project you would have to undertake. My build sounds slightly different than smurphny's. On #54, there is solid laminate under the mast step (no core) running the full width of the cabin-house and the mast wiring runs through a fitting on the deck on the stbd side of the mast. The supporting structure below is 12 teak stanchions. 6 on each side of the bulkhead that support a pretty massive white oak beam. So, I guess the main bulkhead could be considered semi-structural. The beam was in good shape as were the bulkheads. I had a very small amount of perimeter rot where one of the chainplates had leaked a bit but it was easily repairable.
The formica bulkheads are a great surface for painting. I sanded mine with 220 and painted them with with 1-part polyurethane. I also re-varnished all the interior mahogany. Looks great and was a relatively quick and easy project. I took inspiration from smurphny's composting head build and built one myself. That was one of the best upgrades I've done to the boat. No more stinky hoses and holding tanks.
As far as sailing is concerned, weather helm was terrible with the original sails. They were a bit blown out. I bought a new main with three reef points and a 135% rf genny. The difference was like night and day. I also installed a new Garhauer traveler which gives a lot more control over the main sail than the old system. Weather helm with the new sails is quite manageable. In under 10 knots, I fly the full main and genny. Above 10 knots, weather helm kicks up pretty steadily. I usually tie in one reef between 10-18 knots and leave up the full genny. It handles quite well with this combination.
If your rigging is original as mine was, that's one of the first things you'll need to look at. All of my swages had cracks. It was and issue of when, not if, one was going to fail. My chainplates were also pretty corroded at the deck penetration so they were replaced. This is a pretty expensive project. Cost me about ~$2000 to replace all the rigging and chainplates including adding an inner forestay for hank-on foresails. Add to that the $5000 for the new sails and I had about $7000 in rig and sails.
I think the A35 is one of the most solid cruising boats you can buy. They don't have the cabin volume of some of the more modern cruisers but I find it more than adequate. I'm 6'4" and can stand in most parts of the cabin. I think the boat you're looking at would make a fine project, but it looks worth more in the 10K-12K rather than 18K considering the lack of systems. Like I said, the PO wanted $19000 for mine and I paid $7500. I documented some of my projects on the Plastic Classic Forum here: Plastic Classic Forum • View topic - Alberg 35 Refit
. I've done a number of projects since then that I'll update soon. Let me know if you have any questions.