@roog , Yes, the Purifi modules are quite expensive. Unless you need all that power, you could save some money by going with a lower-powered Hypex nCore model. The mid-tier IcePower modules may suffice at a competitive price point and some I think were favorably reviewed here. I was lucky to pick up a new IcePower 300A2 on the forum here for a big discount and built that into an old amplifier chassis. I’ll never use all the power it has available.
There are some well-made TI TPA3255 based products available (you may have to contact the sellers for availability) that should be very simple to implement in older chassis.
This is a review and detailed measurements of the JL Electronic's (JLE) Sylph-D200 stereo amplifier module built on top of TI's TPA3255 amplifier IC. Depending on the configuration you pick, the module costs US $130 to $170. You can read more about it here...
www.audiosciencereview.com
www.3e-audio.com
The newer Merus-based amplifiers getting a lot of attention this year are interesting, but I don’t believe there is a long track record with reliability and quality of construction known, yet. I think the bare modules for these available are still suspect for high quality implementation of the circuit. A front-runner will likely emerge soon. However, the one Merus HAT amplifier I recent received does sound pretty good, though it is low-power.
Depending on how much power you need (over 100 WPC with very low distortion will get pricey), I think there are some very good options. Good luck and happy researching!