I have been creating mobile apps for more than a decade. I am an entrepreneur, an alumnus of Apple in Cupertino, and have a degree in computer science from the University of Waterloo. I am the founder of Monolith, a mobile product development company in Waterloo and Toronto. We created an in-car virtual dashboard and replay app for GM, a chat app for kik, and worked with companies from seed through Series A&B through Fortune 50. In everything I do, it always comes back to this: am I making a great product?
Start small and build up. Big companies don't make all their decisions as a collective. There are lots of individual managers who have the budget and the power to hire you. Once you connect and sell one of them, then ask them if they can introduce you to other people at the company. Build their career by making them the one who made the wise decision to hire you, and then impress more people at the company.
Find a niche. If you are highly specialized in one area, then even a large company may not have a choice in who they are hiring. My company has a high specialization in software for testing hearing on mobile devices. It's not a capability that is needed on a day-to-day basis, but it's also an area where we have little competition.
I think you're asking whether you should go PC or mobile.
The answer depends on your interaction model. Think about a messaging app. The user will be interacting with it briefly but many times through the day. This is an ideal model for mobile. By comparison, an app that requires them to type extensive text or otherwise concentrate for some time, is better done as a "PC web" app.
Another consideration is focus. A todo list manager can be a very simple, focused app which is also ideal for mobile, as you need to be very parsimonious when you lay out your screens on mobile. Look at Clear, an ultra-minimalist todo app for iOS. If you compare that to a complex groupware app like Asana, you can see that the extensive feature set means that it works better on PC rather than mobile.
If you want to talk more, we can have a call.
You need to provide more information about what tools you are currently using.
For the purpose of creating wireframes, I would suggest using Balsamiq, and avoid doing any styling at all. This is good for the phase when you are working out screen flow and layout.
If you are doing a mockup, i.e. something that can look like a screenshot and will determine the graphic style of the app, then you really need to work with a graphic designer. Would you ask a non-programmer to write code? It's the same with graphics -- there's no substitute for a professional. At the stage you're at, you should be able to find an independent freelancing designer quite inexpensively in any decently sized city.
Since this is really a coding question, look at the ultimate resource:
From a review standpoint, Apple will accept your app provided that the functionality you have implemented already is functional and free of obvious bugs. Assuming you've achieved that, then this becomes a business question. You've made some assumptions in creating your app about what users want. Is it better to get the app out to real users and find out if those assumptions are correct? Or would you rather make sure your assumptions are really polished even if they might be wrong? To me, the question is rhetorical.
I would say that you need to get a job, and that job is your startup.
The key thing is that you say that you are at the point where you can get some cash. Based on the attitude I've seen from angel investors and incubators, once you start to raise money it's essential that you're full time on your startup. In fact, you need to have at least one co-founder who is also full time. If you have access to some capital, get it and use the minimum possible to support yourself while you create your MVP.
If the MVP fails, pull the plug and get a day job. Entrepreneurs are dedicated and therefore very employable. You can work on your next idea while you work a regular job, and then again, if you get to the point where you can raise money, go full time on the next startup.