I know that the instructions for the Model SEP say that you cannot use the 5305 SEP if you maintain any other qualified plan. But if you are terminating a qualified plan and then establish the SEP, is there a problem using the model, or must the client go to a prototype instead?

Although the form is not that clear in regard to your issue, there may be limitations under Code Section 415 that may need to be in the SEP plan since the terminated plan's provisions no longer apply.

IMO, the "currently maintain" language should be looked at on a plan year/taxable year basis. If there is any overlap, then I would suggest using a prototype. They can be integrated to avoid discriminating against higher-paid participants, by giving them more!