I was looking at a 03 Trek Navigator 400 to do some riding any thoughts on this model?

He wants $100.00 for it is it worth it?

What is the main kind of riding that you'll be doing? Commuting, recreational rides? City streets, paved or gravel paths?

If the bike is in good shape (no dents on the frame, no rusting, shifts properly, tires are in good shape and not cracked, wheels spin straight and don't wobble), then the price sounds fair, but you want to make sure that the bike suits what you'll want to be doing, and most of all...that you enjoy riding it!

Will be doing bike trails...

It's a comfort bike, with upright geometry for recreational and leisurely riding.

It's also a Trek and not a big box store bike so for $100 I would say yes, it's worth it.

I've got a Navigator 200 and it's a bit heavy and slow, but it will get you anywhere you want to go, and it is comfortable.

That bike is not designed for trail riding. About the worst conditions they are meant to be ridden on are unpaved roads. Some comfort bikes look similar to mountain bikes and others more closely resemble cruisers, but they aren't meant to be used for any rough stuff.

Packed dirt or paved trails, or more rugged terrain? Like MadCityCyclist said, I don't think it would work on rough terrain, but if you mean paved bike trails, or well-worn dirt trails, I think it should be okay.