Is it viable? Short answer is, it depends. If you're going to pay to have everything done, probably not. Rural town, probably not. 4 bay self serve with no automatic, probably not. Complete build for $300,000? Probably not.
You aren't going to get a $300,000 loan for a car wash. You're going to need about a third of that up front.
What are the property taxes on the land? Don't forget to add that to the $1,800 a month debt service.
30 year note on a car wash? You're crazy.
You can easily get a loan for this, even with zero out of pocket... but not from a bank.
Contact your local title companies and ask them to recomend any "private lenders".
I'm only licensed in WA State, but we do about 40 loans like this every year using private lender funds to close loans that conventional lending rejects.
It would be set up with the funds you need for construction kept in an escrow account and paid out in draws as work proceeds, like a construction loan.
Your owned property would be collateral for the loan. As work progresses and property value goes up, the lenders "Loan to Value" would be protected because the only lender funds at risk are those that are going into the improvements of the property.
Private lenders love long amortizations because they get less of thier own money back each month, meaning more of thier money earns interest longer instead of getting paid back to them.
If your property is worth enough, you could even get this done with zero out of pocket for closing costs.
No private lender would want to carry the loan for 30 years so expect there to be a balloon payment.
When contacting a potential private lender, you could provide 3 different lending scenarios that your comfortable with and let them choose.
One example would be:
300k at 9% interest, amortized over 30 years with balloon at # years. (keep the balloon at 7 years or less)
For this example your principal and interest payment would be $2,413.87
This should be considered a temporary loan untill you get it built and refinanced.
This type of loan is a "tool" to get the job done, consider the higher interest rate the "cost" of that tool that you couldn't get the job done without.
You could also offer one year garaunteed minimum interest in exchange for a lower interest rate. (keep that in your pocket for negotiating)
Run your numbers and make sure everything pencils out.
Also, talk to your accountant about what you are planning so they can help guide you into the best tax strategy. Accountants hate it when you come to them with "look what I did... now what?"