Sorry for the loss of the Trek 400.
My guess would be an aluminum Domane the AL 2 is where they start. at about $1000 US.
Let the Forum What part of the world are you in and what size would you be looking for and I bet you would get lots of classic/vintage suggestions.
I'm in the USA, and the size of the bike can be in the medium or large range. He's rode my medium hybrid, and large hybrid with no problems. I figure if we went new we can get him fitted at the store. When I went to get my mountain bike I tried 2 sizes and was able to find the one for me.