For the m54, get the aluminum thermostat housing over the plastic one. Also the sensor on the lower radiator hose has a small o-ring that tends to leak very often. Order a few of those o-rings.
The plastic tanks can be a mixed bag, I had to send one back that was leaking, but this one has been good.
You can drain the block out, there's a 13 mm plug on the exhaust side of the block near cylinder two. You can get to it from the top but it's blind, it's a little tricky. Underneath isn't much easier. The small quarter inch ratchet works. If you decide to open it order a replacement plug and washer. Another solid gallon and a half a coolant will come out and it will drip everywhere, either a lot of time with the rag or pressure wash is needed. If you have a big enough drain pan you can get most of it in there. But cover the floor with as much cardboard as you can find before you start, even the radiator is hard to get clean.
I never did a flush on these cars, just a good drain and vacuum fill with distilled water. Drive it for a few days. Drain again. Repeat two or three times until it comes out clear. Then fill with the nice blue before it gets cold outside.
I used to do a flush on the Honda, just put in the garden hose up to the thermostat housing and lett it gush out of the upper hose. You could probably do that, but you still would want to run with water for a few days I think. You would also have to jump the connector for the heater valve to ensure that's opened. But you wouldn't have to drain the block.
