Very few stock radios have aux input, you can get an AUX input for honda's here
The cheapest screen is going to be a lilliput. Theres a group buy on here for $240. Anything thats not VGA will suck for GPS. Search the forum, I'm sure someone else has mounted one of these in an accord.
Given that this isn't for yourself, I would recommend you go for stable over the absolute cheapest and get an OPUS power supply.