for hw expert designers, a Frankestein GFX CARD for msx

C64 runs at 1 MHZ, so MSX at 3.xx should be sufficiently fast. I think you'll run in to very nice challenges from an electrical/logical point of view. Why not the 8564/8566 VIC-IIe? This is the chip of the C128 and it was designed to work with a Z80 because the C128 could run CP/M. It also has a C64 compatibility mode.

i've seen that the VIC-II issues a busrq on the main CPU, and this could be a problem. maybe having some kind of fast ram and doing some time sharing (and buffering) could allow simultaneus access of both CPU/VIC like it happens with the ADVRAM ?

You can't compare clock speed between different architectures like that. They have different instruction sets that don't do exactly the same thing in the same amount of cycles. a Z80 at 3MHz isn't necessarily 3 times as fast as a 6510 at 1MHz.

Yeah the architecture can be an important factor, especially if RAM is shared with the VDP, this introduces wait cycles to the CPU.

What also matters is how many cycles it takes to execute instructions, on Z80 instructions take 4-23 cycles (5-25 on MSX), whereas on the 6502 they take 2-7. But the Z80 has more registers and complex instructions so it needs less instructions to complete a task. All of this makes it difficult to compare.

For a BASIC speed test, an additional factor is the efficiency of the BASIC interpreter implementation, this varies wildly and makes it difficult to perform raw CPU speed tests with it. But in practice (in assembly), I think probably an MSX and C64 are similar in terms of processing speed. At least I’ve not really seen strong claims that the one is significantly faster than the other.

If the goal is to experience the C64, buying one is way better: you also get a very good sound chip with it. If you just want the looks, perhaps changing the color palet will do. If its for the challenge: I think this will be a big one.