Ainge should continue to target the best players he can for his goal, which he has stated is long term success.
If IT ends up needing surgery and missing a portion of next season, then I would think this guarantees that we keep the pick and take Fultz. No sense in trading for a win now guy if one of our win now guys is hurt.
The Celtics should absolutely target free agents if they help our team, short term or long term. The thing about free agency is we add these guys at the expense of cap space, cap space we would need to use anyways. I know a guy like Hayward would require some other moves on our part, but most of those guys aren't in the short or long term plan regardless.
I am one of the posters who feels our future is in the hands of our youth (Brown, Fultz, Brooklyn 2018, etc...) but I fully expect Ainge to try and put together a winning club in the years before those young guys take over, especially if he can add players in free agency.
If anything this might alter his plans with IT most. If Isaiah misses time and Ainge feels comfortable with other guards he might not see resigning IT as a high priority.
As for Horford, I believe the Celtics targeted him for a reason. A very solid big who can run an offense, hit threes, great passer, provide some d, and an excellent locker room guy to boot. The market established Horford as a max contract guy and the C's (as well as several other teams) were willing to pay it. I would put Hayward in the same category. I don't see Al going anywhere, but other vets like Bradley and Crowder could be shopped, especially if they are looking for big money that might cut in to Ainge's long term outlook.
Ultimately the Celtics want to build a winning team but I don't think they want to sell the future to go all in for the present. You never know what could happen to a Cleveland or Golden State during the season. One major injury and a door can open, so why not put together a competitive team?