I've had your exact same quandry finding a promising replacement at the end of the season. I ended up rolling the dice and it worked out in my favour both times. One happened to be a keeper who made a huge difference. GK ratings went from 5-6-7 to 7-8-9s straightaway. If your keeper is mediocre it will cost you more than at any other position. Head to head my old keeper who was around 125% was doing worse near the end of the season than my new scout keeper who went down to 95% the following season.