I was previously denied a student visa due to earlier ESTA overstay and a bar when I was 19... The years went by and when I met my now husband and we decided to file for K1 this worried me as well. Other than being completely honest on the DS-160 it wasn't a problem. At the interview I was asked why I had never visited my then fiancé in the US (he visited me) I said cause I didn't want to go through the tourist visa application process since I was ineligible for ESTA and was previously denied a student visa. He then asked me how come I was ineligible for ESTA (although it was right there in his papers lol) So I said I overstayed years ago.... he asked me how long, what was I doing and how did I support myself during my overstay. I was just honest about all those answers and he said as long as my statements checked out with their systems my visa was approved.
I know you guys don't have a past overstay, only the visa denial.. and as Penguin explained you are now trying to immigrate so it will not be a problem, unless you are dishonest during the process.
A student visa gets denied if they suspect the beneficiary wants to immigrate. A k1 fiance visa is to immigrate, so there should be no problem with the previous denial, unless he lied to try and get approved for a student visa (such as said he was married when he was not etc).
