What do you do if a potential client is looking for a DJ who can provide the stuff you mentioned? If you agree to do the event, how long would you say it would take to get that stuff?
There's quite a bit of songs we as a DJ have that we may or may not play. You never know when someone may make a request for a specific song and specific version. Best to have it and need it then not to.
Don't forget nowadays you can easily go online and music on the fly if you have the way to do so and know how to do it.
Getting the music is easy, and takes very little time. Acquiring the knowledge to discern the right stuff is what will make or break your results.
While I was still a very young DJ I was asked to do an event that would be all 1930's and 40's music, big band and crooner type stuff specifically for dancing. I was at the time a club DJ and Video DJ playing current hit 70's and 80's dance/R&B. I spent at least two weeks in a studio listening to music - using my dance floor acumen to discern which of the tracks I was hearing would give the energy level I wanted while also corresponding to specific ballroom dance steps that the older crowd would be doing (Foxtrot, Samba, Rhumba, Swing, Waltz, etc.) I couldn't pronounce half the conductor's names at the time - but the result at the party was that people thought I was an expert in the genre!
The key thing to remember is that the party is only 4 or 5 hours long, while each track averages 3 minutes. So, I only needed to have a maximum of 100 go to tracks for my own piece of mind - along with specific requests made in advance by the host. The "investment" here is not the music - it's the knowledge that pays dividends in a DJ career. Trusting the client's requests and taking the time to research was well worth it. It made me a far better wedding DJ in the years that followed to know how to get the energy going in a room during the times most DJs write-off as "background music."
I once auditioned at posh top of the city lounge/restaurant where they wanted to add a DJ but keep the sultry ambience in tact. This was not to be a "club" or "party" and they assumed and expected no dancing with this format. What they wanted was smooth and sophisticated atmosphere. Each of the half-dozen DJs who worked the nights before me indeed had no one get up to dance, though people thought the music was "nice." I on the other hand got people dancing within 3 or 4 songs and people moved on and off the floor all night. Having had years to build and expand on the knowledge of those two weeks spent in the studio I was able to put out a more intoxicating mix that while remaining sultry and sophisticated - also enticed couples to stay and dance.
I still use that build process today. My music knowledge & experience is much wider I only listen to requests I don't know. I pour over every request list given to me and if I don't already have the tracks I add them. So, I basically continue to build my library on the basis of what people actually ask for along with a very modest CHR pool subscription. What use to cost me over $3,000 /yr to maintain now requires that I spend only about $225 /yr.
If I were a younger DJ today I would not set about building a music collection. I would build an audience and let them build the collection for me one request list at a time, while also moving forward with a modest pool subscription of CHR. If all of this can be done with a streaming pool that would be even sweeter. It seems to me that in this day and age "everyone is a DJ" or at least has the potential to be.