Well first off, welcome to the forum and there will be just as many opinions as there are units out there. But take them for what they're worth.
My suggestion is to go and look at some units, new or used doesn't matter to get some ideas of floor plans and options. Sit on the toilet, lay in the bed, watch tv, and go through all the motions that you would if you were staying in it. That will give you some basics.
Look at the GVWR sticker on the unit and make sure you can tow it at it's max determined weight capacity. Narrow it down a little before you get too involved with some extras. Take a pad of paper with you, take lots of notes and write down the do's, dont's, pro's and con's. Consider how you will cmap will be a big part as well, hook up facilities all the time, dry camp, etc. In addition, where you plan on goinng some times has limitations on size.
so in short. Go to an RV show, selling lots, etc and get some good info. Then go home and poke around on the internet and look at floor plans, weights, capacities, lenghts etc. Please for the sake of yourself, DON'T LET SOME SALES PERSON SELL YOU A BILL OF GOODS!!! ESPECIALLY THOSE THAT SAY, "OH, YOU CAN TOW THAT". Just be careful and if you know someone in your area that is willing to go wiht you and help out a bit, get the help. Reach out to those in and around, they will be happy to help!!!!! Here on this forum and other forums, manufacture specific forums too. These forums will also give you some good info. For us, we have had bumper pulls and now fifth wheel, we love it and enjoy it. On our second Grand Design forst the 303RLS, now 337RLS, but there are little things that I like that kept me wiht this brand/model. GOOD LUCK !!!!! Enjoy!