2005 G35 Sedan (AT) 58K

I'm relatively new to this forum but have spent quite a bit of time searching through it and I can't seem to find a thread involving these two specific codes...here's my issue:

Last week I stopped at an intersection and when I pushed the gas the car didn't move and the word "Slip" showed up under the D on my dash. Yesterday I stopped at a stop sign and when I pushed the gas the lights on my dash and shift console started flashing, the car was slow to engage & there was a grinding noise (sounded and felt like I was driving a MT and struggling to get it into gear).

I took the car to my local mechanic and he found codes P1815 & P1705. At which point he typed the codes into his computer program, said "this isn't good" and proceeded to tell me that the most common fix amongst his network of mechanics was a new transmission.

Long story short, I just bought the car and am hoping it doesn't need a transmission. Any info, advice and/or guidance from members who have had a similar issue and/or encountered these two codes simultaneously would be greatly appreciated!!!