Hi,
A quick scan over the documentation on those two sheilds (I presume you mean this CAN bus and their new fi250 WiFi shield) they've covered themselves in glory for summarising the pins haven't they. They're not even up at sheildlist.org. Nevermind.
It appears the CAN bus shield uses the SPI interface, therefore on a Uno that's pins 13, 12, 11 as well as a chip select pin which in their example code seems to be D8. The WiFi sheild uses serial for communication and they recommend the hardware port, therefore D0 & D1 so there's no clashes there. So in answer to your first question, if my presumptions are correct the two shields will live happy together on your Uno.
As for the phone application it's pretty straightforward to construct apps for Android on several free sites. I like the MIT App Inventor but you may find others you prefer.
If you are going to interface with a phone, WiFi probably wouldn't be my first choice but that's up to you. MIT's app inventor has support for Bluetooth but not WiFi as far as I recall. Other alternatives might do so.
I wouldn't write yourself off for doing this yourself. This is far from rocket surgery - and you sound like you've got a firm grasp of the harder part of the project already.
Cheers ! Geoff