There is a reason a Class B sells from $60,000 to $140,000 new. Go out and start shopping for a used B - you may find something that is 25 years old for around $20,000 - maybe. To that you will add several thousand in repair or replacement parts and components - if you get your friend to do that work for you. Go out looking for a small C and you can spend a lot less. Go out for a travel trailer without the car or truck to tow it and you will be in your price range.
It sounds more like you will undertake this DIY project and lose a lot of money trying to put together something that you - or your friend - don't know enough about to complete. Not being negative - just realistic.