My own $0.02
First of all, you can tell by my avatar that my wife and I are still sailing (33' Pearson Sloop) and for the past year or so have been discussing and planning to liveaboard in the not too distant future.
We still enjoy sailing and innitially we started looking at larger, roomier "beamier" sailboats. Ultimately and I suppose like many others, we soon came to realize that for the price of that bigger sailboat (one comfortable enough to actually liveaboard) we could do just as well and be much more comfortable with a trawler. We do plan to keep the Pearson though, at least innitially.
So here we are, Reading everything we can to get knowledgeable on the subject, discussing our current lifestyle and imagining how it would translate to living aboard and being realistic about how we would actually be using the boat.
Then of course you start factoring in all the possible boats that there are on the market. The various styles, types, propulsion, displacement. Incidentally, we are not interrested in investing hundred's of thousand's of dollar's in a new or nearly new boat, Our market is going to be the "used" market and that less than $120K.
It's amazing how many trawlers there are available but were noticing that the really good values do not last on the market for very long...
I'm not even going to discuss the options, feature's and variation's that are available but I do most heartily agree with one very important piece of advice we've received. COMFORT. I don't necesarily mean luxury, I have nothing against it at all. But comfort in that being something that does not detract from the quality of your daily life.
I'm not sure where one crosses the line between being considerred a cruiser or a full time liveaboard. Most everyone is willing to endure some sacrifice in comfort and consider it as some part of the price to be paid for the result or experience. But I for one cannot afford, nor have the time to be buying and selling multiple boats to find the "right one". Best we can do is to look at, get aboard (experience) as many vessels as we can and start compiling our selections/desires from that.
I think boatpoker said it very well: Go with the smallest boat you can be comfortable with. Word's like smallest and comfortable are subjective though, defined only by the one who has to live with the decision.
So far the only thing we've ruled out is anything under 36'. But whatever the "right one" turn's out to be, it WILL have a sundeck!