Only downside to starting with this is that it's peak classic Castlevania and everything else (while still great) will probably not feel as good.
But yeah, Rondo is amazing and my favorite of the classic era, and thus pack is one is ridiculously great value. People will tell you that the included version of Symphony is heresy and that the borders are Satan, but honestly... It's a perfectly fine version of the game, and in many ways arguably the definitive one. Sure I'd prefer to introduce everyone to the game with the original dub, but it's a very small detail and the new one is objectively better, Lowenthal Alucard aside lol.
Also peoole despise the Rondo remake, and while I agree that it's inferior compared to the original, it's very much worth a playthrough if only to see the differences and added content, which I knew nothing about playing it for the first time because no one ever talks about the game! But yeah, it's awesome, framerate is worse, music isn't as good and much of the finesse of the original's visuals is lost with the new look, but it's still a great game, so I encourage you to go back to it once you're done with the original.
As a side note, I'm sure there are a couple technical flaws to both OG Rondo and Symphony in these versions and people will probably tell you they make them trash, but believe me, you won't ever notice them lol.