Yea I guess you could iso pre, but I don't mind the call. Also, UTG limpers typically have low PPs and don't fold to raises that often so you might be building a larger pot than you want.

I think I would raise the flop more. This player is a donkey and you want the money in as fast as possible. Get it in on the turn when he spazzes out. I wouldn't worry too much about giving him a range here.