Previously overstayed and now applying for a Fiance visa

Hi everyone,
I am applying for a fiance visa so I can go to UK and marry my partner. I had overstayed my visa before I came back to Pakistan. I did that voluntarily and on my own expense (apparently this counts too).

My partner lives in a rented house and works part time. She does receive benefits as she also has son from her previous marriage.

I will be travelling to UK and have enough money to stay for 6 months +. We had been planning for me to stay with her.

Will my overstaying, her being on benefits or me staying with her (as she gets benefits for rent) create any problems?

I would really appreciate your guidance, help and advice in this matter as I am dying to be with her and my step son.