If you are short on time and budget, you should consider cross-platform development, as you won’t need to have separate apps for iOS, Android and Windows Phones. Despite the fact that such apps have a few restrictions, the cost and time savings are significant.
There are plenty of technologies to build such mobile applications, but we use the most stable and popular - Cordova (formerly called PhoneGap).
UI/UX is probably even more important for mobile apps than for web development, because you have to put all the functionality from a big screen into a small one. We have successful cases of creating such experiences for mobile apps which are currently on the market.