In terms of the complexity of the app, I'd say to start looking into the kinds of technologies you'll need to fulfil the requirements. Sounds like you will need a server somewhere with an API, and of course the app itself on whatever platform is appropriate. I have not done much mobile app development myself, but I would not underestimate the impact on the project of, for example, learning how to develop an Android app. Each ecosystem has its own vocabulary and conventions you will have to understand in order to develop the app well.
That, of course, is not to say that you can't get something out in two or three months that is functional, if rough around the edges (at least in terms of the code).
If you're really worried about the possibility of not being able to deliver, I would advise you to pay close attention to the terms of the contract if they offer you the gig -- how much freedom do you have on the project, are you accountable for its success (particularly in a financial sense).
You can recommend that the company bring on another developer, perhaps before you sign stuff. Treat it less as a negotiation with an employer and more like a consultation with a client: you want the app to be successful, and another developer would help to ensure that.
That, of course, is not to say that you can't get something out in two or three months that is functional, if rough around the edges (at least in terms of the code).
If you're really worried about the possibility of not being able to deliver, I would advise you to pay close attention to the terms of the contract if they offer you the gig -- how much freedom do you have on the project, are you accountable for its success (particularly in a financial sense).
You can recommend that the company bring on another developer, perhaps before you sign stuff. Treat it less as a negotiation with an employer and more like a consultation with a client: you want the app to be successful, and another developer would help to ensure that.