There are some basics like avoid rust, obsolete engines & transmissions and bald tires all the way around, but there is no one easy answer. You are the only one that can determine what is "right" for you and your needs. Read as many of the "build" threads as you can cram, set some parameters for how you intend to use it, take notes and you will put together a picture of what will best fit your dreams, budget and skill level.
Then...dive in. That is pretty much what everyone here did or we would have all just bought off the shelf RV's.
Best of luck and go for it.