Most everybody is going to say "Get a DS1054Z 4 Channel scope, hack it to get to 100 MHz and it includes decoding for serial buses". I haven't done that but I probably will and the procedure is well established.
Watch Dave's videos on the 1054 and see what he has to say. For goodness sake STAY AWAY from the negative threads on this forum. There are some truly vitriolic haters on those threads. You will also see it in just about every thread that even mentions the word 'scope!
Well, to be fair there's some basis to the critics of Rigol:
https://www.eevblog.com/forum/projects/project-yaigol-fixing-rigol-scope-design-problems/That doesn't render the scope useless, but it is a pretty stark display of incompetence in terms of designing hardware properly, and a prospective buyer should be at least aware of the problem and what's required to fix it.
Rigol is pretty good bang for the buck at every level.
No, certainly not at every level. Rigol offers without doubt the best value for money at the bottom end (DS1054z), and the DS2000A has been similar value for money back then when it was only competing with the Agilent DSOX (but these days there are more alternatives), but the DS4000 in general isn't (again, there are other alternatives these days), and the DS6000 is pretty much a bad joke with a ridiculous price tag.
There are others such as Siglent and on up the food chain to Tektronix. The cost just keeps climbing.
Yes, but Siglent, while producing good hardware, has a long and very poor track record with their firmware quality. It doesn't mean the scope won't do what the OP wants it to do, but it's important to be aware of the limitations and issues, and as nctnico says, if it doesn't work today don't get fooled into waiting for a fix, return it.
It also shows that in general you get what you pay for, although Tek (the once great brand that gave us the best analog scopes but since the advent of DSOs has only come up with mostly mediocre at best products, and today it pretty much represents the bottom-end of the big brands) shows that this isn't always true.
My advice (not that I expect anybody to pay attention to what I have to say), buy the DS1054Z, learn what digital scopes can or should do and, later on when needs change, buy something else and sell the 1054 if needed.
I fully agree. At a budget of $400 the DS1054z offers the most bang for the buck. The 2nd hand market often has great deals but at $400 there's pretty much only "untested" (i.e. known defective) digital stuff, some old sampling scopes (useless for everything except some niche tasks) and some analog antiques (which realistically shouldn't cost more than $50 or so). So yes, the DS1054z is the best option here.