The best way to shop for a Class B or any RV is to go to an RV Show where there will be multiple manufacturers of what you are interested in represented. You will then see all of the makes and most of the models that you can walk around in and see how comfortable you are.
If you are thinking about buying a van and going to a custom shop to have them turn it into an RV you will be paying a lot more than having a Class B manufacturer do the work and you choose the options. Roadtrek lately has said that they will customize - depending on to what extent one wants the interior to be customized. Sportsmobile as mentioned does custom interiors.
Go out and see what there is - you may find out immediately that you don't like traveling in something this small or you will like what you see. Then, too, if you want to buy pre-owned instead of new you will know what to search for.
As to prices on new Class Bs - expect to go into shock. You can get a small A or a good sized C for less than some Bs. Making it smaller costs more.