It depends. Have you seen any before? Have you had your resume brushed up? If so then no you don't need one. The Spirit online application is nowhere as involved as Airline Apps, FedEx, Pilot Credentials, etc. where it really pays to have someone to go over your app with a fine tooth comb. So not worth the ~$300 IMO for just an app review for your Spirit application. Although you may want a resume review if you haven't had one.

As far as brushing up your interview skills there are none that I know of that specialize in Spirit. But if you are looking for preparation on TMAAT and other interview questions you can't go wrong with any of them. Cage and Marshall, Check and Set, Emerald Coast, etc.