how do i talk the mother inlaw into leting my fiance buy this? lol

found this on craigslist my fiance really wants. we are thinking of taking out a small loan to buy it. the problem is her mom just helped pay to get the jeep fixed. the jeep is still technically her moms but we are the only ones who drive it. so i can for see her getting mad because we dont need a third car and the jeep just got fixed and where will you park it blaw blaw blaw.... lol what are all the good argument points i can use to help her get this car? lol