When I started to explore classical music in a big way about 3 years ago, I searched out the Naxos label because of the number of performances (over 2000) and the inexpensive price. After a while though, I found that there were better performances of the classic composers on other labels for a little more money. Also, whereas I am sure that I could come up with 15 to 20 good sounding Naxos releases, they would wind up being of more modern, not so well know composers and I am not sure if that is your father-in-law's cup of tea.
With that in mind, I am going to suggest a different approach. In the last year or so, the RCA Living Stereo Series of great performances from the late 1950's have been remastered. They have been reissued in SACD hybrid form (playable on all ÇD and SACD players); offer 3 channel sound (FR; FL; CC) ... a plus if your father-in-law has HT; and sound really great. You could get 10 to 12 of these performances for the price of 15 to 20 Naxos recordings.
If this sounds promising, here are the dozen that I would recommend:
Beethoven - Symphonies No. 5 & 6
Beethoven - "Violin Concerto in D"
Chopin - "Ballades"
Franck - Symphony in D
Mahler - Symphony No. 4
Anna Moffo - "Arias"
Ravel - "Daphnis Et Chloe"
Stokowski - "Rhapsodies"
Strauss - "Also sprach Zarathustra"
Tchaikovsky - "Pathetique" Symphony (#6)
Van Cliburn - Rachmaninoff Concerto No. 3
Van Cliburn - Tchaikovsky Piano Concerto No. 1
These are available mail order from Tower Records or J&R Music for usually $10 a CD. Music Direct has them packaged and specially priced by when they were released (there have been 3 separate releases). If you do want to throw in 2 Naxos releases for good measure, get the new recordings of Brahms' Symphonies Nos. 1 & 2 with Alsop conducting.