In my opinion, there are two approaches.
You know the saying "Buy cheap, buy twice".
I say why not (buy twice)?
If I have no idea about something reasonably complex, I always buy a cheap model first.
If I see that it's not enough, then I know why and I know what I need and what I have to look out for.
In the end, you might even save money this way. New 100MHz scopes are available for well under $200, which is significantly less than the price difference between the Rigol MSO5074 and the Siglent SDS804X HD.
Others (especially salespeople) say: "Buy something real straight away".
Often you end up with a device that is too complex to start with, or where you simply don't need 90% of the features.
I know that many people here have a different opinion.
It's a question of personal preference.
However, the Siglent SDS804X HD is reasonably inexpensive, so you could give it a try.