If you were financing $450,000 with a 30 year loan at a 3.5% fixed rate your monthly payment would be about $2020.70 for the principle and interest. You will also have property taxes, home owners insurance, and possible HOAs to pay as well on top of that so it depends on those things as well.
I would suggest talking with a few lenders and see what your options are. A lender should be able to tell you whether or not you could qualify for the loan and how much you will need for a down payment. There are zero down loans, down payment assistance programs, etc. A good lender should be able to explain all of your options.
It really depends on a lot of things. I would have your husband talk with some lenders and they should be able to tell him if he can qualify or not and if he can't qualify right now they should be able to let him know what he can do to improve his credit.
Selling your current home and purchasing another home at the same time isn't the easiest real estate transaction to do on your own. I would really suggest that you talk to a few real estate agents in your area and see how you feel. You can do this type of transaction on your own however you will usually benefit from working with a professional.
If you contact Zillow they should be able to help you with this. When I have spoken with Zillow about advertising in the past they have suggested that I team up with a lender to split the advertising costs so you may want to find an agent who is currently advertising on Zillow in your preferred area and talk to them about partnering up with them.
One thing that you can usually do is require that any buyers be pre-approved with a lender that you identify. They don't have to use that lender for their loan, they just have to be pre-approved with that lender. Your agent should be able to help you with this.
If you had an inspection contingency written in with your offer it should say what your options are and what the timelines are for those options. A lot of times you can cancel buying due to repairs needed after you've done your home inspection. I would talk to your agent ASAP and let them know what your intentions are.
I would say that having 8 showings since October 4 isn't bad. There are many homes that don't get showings at all. I would consider waiting a little longer to see if you get any offers and if you don't then you can talk with your agent about possible reducing the price a little and see if you get any increase in interest and hopefully an offer.
Out in my area you can pay somewhere around $325.00 for a smaller home and the price goes up from there. Really depends on the inspector, type of home, and size of home to name a few things.
The accuracy of Zillow Estimates vary by location. Zillow uses a series of algorithms to determine their Zestimates and sometimes they are right on and other times they aren't even close.