1994 Nissan Maxima Won't Shift into 2nd.

I have a 1994 Nissan Maxima with an Auto Transmission. Engine VG30(E) Transmission RE4F02A. The car runs fine except it won't shift into 2nd. It reverse's perfectly fine. It doesn't feel like it slips at all when it's in first, it just has a top speed of about 30 if you really rev it.

Upon draining the transmission fluid it was darker and smelt a little burnt. I have pulled the transmission however I don't want to waste time and money. Some people are saying if I go in replace the filter and put new transmission fluid in it then it'll work the filter and bad fluid was causing incorrect pressures. A shop said I need to replace the whole transmission it's bad(without much detail) Which should I do and could it also be something electrical/computer? Please be detailed about why something might work over another.

Thanks!