Welcome to BP!
In this transaction, I strongly suggest you not get "creative" !!!
Use a standard note and deed of trust prepared by an attorney.
Make sure the buyer has the ability to pay, income to debt and job history. Get a credit report, but tell the borrower it's a formality requirement, not an underwriting matter so much......but, your buyer needs to be free of judgments and pending legal notices as your seller could come up in a second position. Have your attorney do this.
I'm assuming the property is owner occupied and the seller is exempt from Dodd-Frank.
Get a mortgage servicer involved to collect payments and the loan servicing requirements, the seller will get a check from them that shouldn't bounce! LOL
Yes, it's 250K down, balance financed. Having the payment date on the day of settlement will make settlement easier.
Might consider too how much the buyer has left, it is not a good idea to allow someone to dump all their money down, regardless of the temptation to take it, they need reserves.
Compute closing costs, you may need to net out costs and reserves and finance a bit more, that's a better way to go.
Also, have your broker look this deal over! Good luck :)