Thirty grand is awfully low for the kind of boat you describe. Each of your requirements--- safe, comfortable, durable, and easy on fuel--- comes with a price tag.
Lots of questions for you to answer. How big of a boat do you need? Living aboard generally benefits from more space than just cruising. How many people will be living aboard? Where will you be using the boat? Maine, Chesapeake Bay, southeast coast, Gulf coast, southern California, Pacific Northwest, Great Lakes, rivers? Each one of these has its own set of requirements that can influence the decision of which type, make and model of boat is best.
There are a lot of books out there that will offer advice as to what kinds of boats are suited for different purposes. I've never read one as we got into this kind of boating in an evolutionary way, moving from smaller boats to what we have now. And while I was not a boat owner until the 1980s, I've been around them and people who had them since I was a kid in Hawaii. This didn't make me an expert but it at least gave me a general sort of knowledge about boats and boating.
Probably the best bet at this point is for you to do some serious hanging around at your local marinas and look at boats and, more important, talk to people who have boats that appear to you to perhaps be the sort of thing that would meet your needs. Most boaters are more than happy to talk to you about their boats (I'm not but I'm not the norm in this regard
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One source of information about the pros and cons of a lot of makes and models of cruising boats is David Pascoe's website. He has also written one or more books on the subject that you can order from his site. He is very biased and very opinionated and his opinions don't always jibe with reality but he does offer a lot of good information about a wide range of boats.
But from your post, it seems to me you need to learn a lot about boats in general, and talking to boaters in your area is probably the best way to do this. You can also charter a boat for a week or whatever to see if you even like the "lifestyle." As I get the impression you have little or no experience in cruising, you most likely will not find a charter company that will rent you a boat on your own. But you can charter a boat with a captain, or if you know someone who does have a sufficient boating experience, you can share a charter
And I think you best prepare yourself for the need to revise your stated budget upwards. Unless a very small or extremely old boat will meet your needs. And an extremely old boat, particularly for the cost you state, will most likely be a full-time job to get it safe and comfortable, let alone reliably operable.