Which solar panel to buy

[the evil twin]

Please explain what you mean

Very, very rare to find a good MPPT below a couple of hunjy. Good name brand will be $300 and up

As a point of interest the 25 and 40 Amp jobbies in the BCDC series are quoted as MPPT by Redarc but the SRP series wasn't AFAIK

Depends on the guts but a big MPPT Controller will take 60 to 150 Volt panel input give or take.
I 'think' they can get the costs down of the MPPT if they limit the Panel Voltage and it's interesting that Redarc quote a max panel input of 30 odd volts
30 volts is the normal limit for a PV controller ergo those 10 and 20 amp redarc stand alone jobbies may be MPPT but personally I am not 100% sure.

Best way would be to check with Redarc but either way you know you are getting a good controller