A lot of great suggestions on here. I'd definitely say you need to plan on additional expenses upfront such as: permitting fees, extra holding time if there are permitting issues, all the little things in the drawers and kitchen etc, hot tub, amenities such as toilet paper, spices, laundry detergent, dish soap, shampoo and conditioner etc. - stuff you need to restock and have on hand. Also replacement costs of items like if towels get ruined etc put a buffer in your budget! Even put buffers in for minor repairs or other replacements. Software you pay a monthly subscription to for running the STR. Photography. Marketing expenditures. I could go on and on of course there are a lot!
I recently signed up with Minoan which I think is a great service! They have deals in place (like 10-40% off) with lots of places you might buy anything for your STR like mattresses, furniture, dishware, soaps etc etc so you get further bulk-type pricing. Companies like Crate and Barrel, Pottery Barn, West Elm, Bear, Public Goods, oxo, Fellow etc) It's been great! www.minoanexperience.com Great for when you are scaling your business as well.
Good luck!