Gs400DTX

Hey everyone. I got a 2000 GS400 i got for $500. I took it park place lexus to get it looked at and they told me i need a new transmission because the transmission oil pan seal is leaking. Is that true? If so where can i get a good transmission at? Or am i able to reseal it. The tech said it better for me to get a new trans because i will have issues with the transmission after replenishing fresh fluid in an old automatic transmission – they tend to start slipping. please help and thank you.