You can pick a charity at any time to run for... though obviously if you hope to get your FLM place through a charity, then you'll need to pick your charity first and contact them.
As you'd imagine there is an entry fee for the races (around the £30 mark I think), though if you get your place through a charity I think they may have already paid for your place (though of course expect you to raise a certain amount of money for their charity in return).
As for a predicted finish time - that's a tough one if you've not raced before... though if you have completed a race (of any distance), you can use a race time predictor to predict your marathon time from another race result. Just tap "race time predictor" into google or something... If you've not raced then maybe use the calculator but input a training time/distance?