Classical is the arguably the best genre to collect used LP's.
Typically cared for, less in demand. Depending where you're at, Goodwill and record stores that bother with Classical will have gems for cheap.
50's and early 60's are the peak of the LP production.
Do your Classical music history homework. Learn about the genres and eras which will help you understand what it's all about.
Find/discover your top 5 composers and their "must have compositions."
Endless choices.