There are many after market radios that will allow you to keep the steering wheel controls. Eclipse does it, and I think Alpine might too.
Many of those units that do not allow you to keep the steering wheel controls with just the unit itself will have an adapter that you can get to allow you to keep them. Pioneer is one of those brands.
As far as the aux and ipod controls. A lot of the new after market head units come with an ipod connector. If they don't, again there will prolly be an adaptor that you can get. The Aux input will prolly not be needed with an after market HU.