The J45's and 50's coming out of the Bozeman, Montana plant are probably the best that Gibson have made since the first were built in 1942.
Avoid 60's and 70's square shoulder 45's with double 'X' bracing which is a tone killer. 1968 was the last year of the original slope shoulder 45's and you won't go far wrong in either a 45 or 50 (basically identical other than the natural finish of the 50).
Personally I would get a new one; I have three recent Gibson acoustics, an Advanced Jumbo, SJ200 and J185 Koa and all three are fabulous sounding, well-built guitars.