I have a AMT .45 with 5" slide, which IMO is a really nice 1911.
The AMT's earned a bad reputation for the slide rails "galling", but most agree it was due to the SS not being lubricated properly. Modern lubes/greases can prevent this.
Visually inpect the faying rail areas for uneven wear, marks etc., plus cycle the slide to check how it feels.
I use Froglube grease on my rails and have not had an issue.
AMT's also were noted to be good/poor guns as far as accuracy goes. So, if you can, shoot the gun to see how it groups.
The fellow I bought my 5" AMT from had a 10mm AMT long slide for sale at the same time. If I had the money at the time, I would have bought both!!
Depending on what he is asking for it, if it's a good price, I would considering buying it.
PM me if you have any questions.
Hope this helps.