So, I can get a PPL, the do the FI(R), and still be paid for it without having to do a CPL?
You can be an FI, yes. I don't know what you refer to by FI(R). Is that a national UK rating? The EASA rules make no reference to an FI(R).
Could somebody please talk me through the exact requirements (minimums) to become a flight instructor?
The absolute minimums for a PPL(A) holder to apply for an FI(A) rating are:
200 hrs flight time on aeroplanes or TMGs
- of which at least 150 hours as PIC
- of which at least 20 hours as PIC on VFR cross-country
- of which at least 30 hours on SEP
- of which at least 10 hours is instrument flight instruction (received)
- of which at least 30 hours is the FI flight training itself
5 of the SEP hours must be in the 6 months before doing the "pre-entry flight test" for the instructor training.
You must also have done a VFR cross-country flight (as PIC) of at least 300 NM including full-stop landing at two different airports.
Unless you are going to instruct LAPL(A) students only, you must also have the CPL theory.
Then there are additional requirements depending on what kind of instruction you are going to give. I strongly suggest you read the source material (part-FCL Subpart J).
Disclaimer: I may have made a mistake in compiling the figures above!