Take your time and make sure it's the floor plan you want. Spend hours and days looking and walking around different RV's, there is no rush to buy.
As far as expenses go....hold onto your wallet. You will most likely purchase-hitches, hoses, sewer connections, kitchen items, outdoor lights and rugs, along with 100 other items. A lot of this can be done reasonably but it adds up. Don't forget about storage fees, if you can't store at your house. This is not a cheap hobby, but the money is secondary every time I hear how excited my 5 year old is about going camping.