It depends on what you want and like. You do what suits you best. There is no need to spend a lot of money if you don't want to. But you will have to spend some to ride your bike in the winter.
I have three beater bikes for example that are in excellent condition. Two have matching fenders and they work perfectly.
They have not all together cost me the price of a set of fenders. If after the winter you find that you don't like what happens to your bike,then start looking at yard sales and in the trash. I have found a couple of mtb's in the trash that needed nothing except air in the tires. I gave them away.
If you like having one bike that's OK too. It may or may not be cheaper.It depends on what kind of conditions you ride it in.