@Mickmca
Hello,
I have installed dozens of Deco Mesh and have never had any issues mixing and matching different model Deco Nodes. The Primary rule of thumb is the Primary Router or AP node should be the most feature laiden one in the mix so start with the most potent first ... AX6600 -> AX3000-> AX1800-> AC2200-> AC1300-> AC1200
So have fun and mix it up. Just ensure that the devices are all from the "DECO" family.
Thus far I have mixed, M9, P9, P7, M5, E3, E4, M3W, M4, S4, X20, X60. Obviously not all in one location 
Also don't fear the Amazon refurbs, I have had zero failures using the less expensive "Refurbished" units.
As for Extended connectivity from point to point: (this can be between Deco Nodes
)
I have had excellent results using both PowerLine(pairs) and Outdoor CPE(pairs) for Point to Point connectivity over longer distances.
Bear in mind PowerLine must be on the same side of the panel and ... AV600 usually10- 50Mbps, AV1000 usually 10-120Mbps, and CPE usually less than 2Km (un-obstructed view)