Good question.
Comparing an old Planar to a modern Rega is interesting - even a high end Rega like a P9.
Of course they are different but the big expensive deck doesn't have it all its own way.
The older Planars have a warmth, cohesion and swagger to the sound which is diluted on the more modern, crisper, tauter sounding versions. The older TT needs the solid mount, low vibration motor to stand comparison - but so fettled, stand it does.
So based on the fact that as a recent ex P9 user of some 5 years, I don't notice a large downgrade going to a Planar 3 I'd say a modern RP3 is a par for par substitute for the oldie.
For the amp, well I keep saying it, but there has been no progress in sonics because there can be no progress. So a modern Rotel or Cambridge budget amp will match ye old Rotel or NAD 3020 just fine.
The loudspeakers are a different story for two reasons. One, there is huge variation between models at all prices but secondly, I think overall voicing has changed and models today tend to be quite aggressive and forward compared to those of a few decades back.
But generally yes you can do as well today for a comparable inflation adjusted budget. I'd argue you can't do better (excluding digital) and that's because the technologies had already matured in the 70's, let alone the 80s.
As an aside, just for the measurists and because it's historically relevant ;-) I pulled out an old copy of HFN Jan 1980 the other day and read the amplifier group test review.
Martin Colloms did the in depth technical testing and arranged the blind listening panel tests.
All I'll say is that the test included Quad, Sony, Rogers, Exposure, Pioneer and Yamaha models from the tops of the range, and a newcomer in the shape of the NAD 3020. On line inputs....there was no audible difference between the group. Only on phono where RIAA differences came into play was there any real deviation in the panel scores, and not against the NAD.
