What is the bes size and make of downpipe for a 2000 SSEI? And how easy is it to replace a new one myself, any special tools??

You want performance, 3", otherwise 2.5", and that will be larger then what is there now. Keep in mind a stock DP is double wall. Replacing it with a single wall DP will give you a tick that sounds like a exhaust leak. You can use header wrap to quite that down. If the new DP is the same as the old one, then changed it is straight forward. Your likely to break the bolts, so just be ready to change out the bolts. 2 at the top, and 4 at the bottom

Never heard a tick from a dp replacement unless the flex is a single wall pos.

A DP swap will make the cabin louder with exhaust sound though.

My TOG DP drives me nuts. Some day I'll wrap it.