Many FPGA dev kits should work for the project. I would prefer a board with a dedicated audio codec. The Mimas V2 apparently hasn't it, just a simple PWM audio interface. Don't know about the MTE labs board.
Deciding for X or A is also a matter of being familar with familiar with one or the other toolchain. Personally I'm biased to Altera.
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Atlys and DE2 are both equipped with industry standard audio codecs.
Resource requirements depends on your project specification, e.g. storage time for reverb effects. Moderate sample sizes can be stored in FPGA internal memory, for large samples, external SRAM or DRAM can be used. All said boards have sufficient resources as far as I'm aware of.