You're going to get a ton of advice, and probably nothing very conclusive. I'll throw in my 2 cents.
Only tried catamaran sailing 4 times in Dominican Rep recently and was completely fascinated by it.
It sounds like you were on a small cat since you were able to single-hand it with very little experience. The experience is very different on a larger monohull. You actually may not enjoy it. I prefer it, but it's not for everyone.
Decided to to start sailing. I found decent condition 88 MacGregor 26d and I am thinking about bying it.
My advice is don't. It doesn't matter what a great deal it is. Another great deal will come along - I guarantee it. And, this advice comes from a person who jumps into everything full-on. I understand the urge to go for it. I would be the last to dissuade you from jumping into with both feet.
However ...
Is it possible to buy and then slowly learn the craft by cautiosly sailing very close to the land on Atlantic coast in only good weather
Absolutely, but don't. Not because it isn't safe or anything like that, but because it is not a great way to get started. You definitely don't need formal education. Mostly, you just need to get out there. However, a 26-foot boat is not a great beginner's platform. Learn to sail in a dinghy until you can do it well. You don't need lessons, but it doesn't hurt to get feedback from someone who knows what they're doing. Also, read everything you can get your hands on. The combination of dinghy sailing, reading, and a little bit of experienced feedback will get you where you want to be in short order.
How long until you're ready to sail a bigger boat? It depends on you, but I would say not long. The Mac 26 is a pretty simple and forgiving boat, and there are plenty of other boats that fit that description. Some people will say that you shouldn't buy a boat this season. I say you can get there in a few weeks if you are serious and learn quickly.
or the only way to start is get sailing lessons, then get Coast Guard certification etc
Many (most?) sailors have never had a formal lesson. I took a class when I started, but the only thing it really did for me was to give me a rescue boat while I was learning and building confidence. Don't worry about any certifications right now. Preparing for most certifications really does make you a better sailor in most cases, but it's certainly not necessary to get started.
Is 26d sufficient to do close good weather sailing until I learn more
If you insist on starting on a larger boat (longer than, say, 17ish feet), the Mac 26 is not a bad place to start. However, you REALLY should get someone with some solid experience to take you out on it a few times. There are certain boat-handling skills with a boat that size/weight that you really can't safely pick up through trial and error. You don't really need extensive lessons, but get someone to show you the ropes until you both feel confident.
The best thing to do is hook up with a local sailing club. You will find no shortage of people who can give you a huge head start and help you make a smart purchase decision if that's what you aim to do. It'll also give you the opportunity to spend some time on a variety of boats so you can see what you like and don't like.
What are first things to check when buying the boad.
At the risk of sounding glib, the first thing to check is your motives. Be crystal-clear as to what you want out of a boat. If you can't do that, you really should wait until you can be. Otherwise, it's kind of like marrying the first girl you meet. You might luck out, but probably not. There is something to be said, though, with not knowing any better.
The second thing to check is your overall impression of the boat. This doesn't take a lot of experience. You can just look at a boat and use your common sense to tell if you think the boat has been well taken care of or not. If it appears to have been neglected, it may still be a good deal, but you lack the experience to know the difference. Better to just skip it. If it appears to be well taken care of, it probably is, but there still may be hidden problems. Once again, you lack the experience to find them, but that's OK.
Some things to look for: Anything that looks structural but is soft, rotting, falling apart, etc. Sometimes what looks bad is not really, but you don't want to get into that. The hull should be hard and smooth inside and out, and the deck and cockpit should be the same. Look for signs of moisture in the lowest places in the boat. Ideally it will be bone dry and appear to have been that way for a long time. Check that the boat sits squarely in the water. Sails should be crisp (that's how they are when they're new). If they aren't, that's not a deal-breaker, but it lowers the value. They should be free of tears. If not, they may need to be replaced. This is expensive, but not the end of the world. The motor should be in great condition and well-maintained. As a beginner, you can't sail a boat that size without a highly reliable motor. All deck hardware should be solid and not show signs of coming loose. "Spiderwebs" (tiny cracks around hardware) are not necessarily a problem. Check underneath deck hardware (from inside the boat). There should be no signs of stress or indications that water has leaked. If there are, examine carefully.
The third thing to check is how it sails. It does no good to buy a boat on reputation. If you don't like the way it sails, it doesn't matter if others do. If you don't have any frame of reference for this, you should wait until you do.
The fourth thing is to get a survey done. If you've already decided to buy the boat, get this done. It'll cost you a few hundred dollars, but it will probably save you at least twice that in the long run. Even if the surveyor doesn't find any deal breakers, he/she will at least give you a great list of things that you know you'll need to have done on the boat. You won't have to figure this out for yourself (which tends to be costly).
