The VW EA888 2 liter is a good motor. The biggest issues are a leaky water pump and carbon buildup from direct injection. I don't know anything about the reliability of the 1.5 TSI EVO engine. These are 2 likely candidates and both are made in Mexico.The cheaper LFP batteries would work better with a range extender since they are more durable compared to NMC. LFP doesn't have issues being charged to 100% continuously, and can be charged many more cycles versus NMC, which is happiest in the 20-80% range, and usually rated for half the lifetime charge cycles versus LFP.
My biggest worry for the EREV would be the reliability of a VW high output 4 cylinder engine...having PTSD thinking of all the oil burning/piston issues of the 1.8/2.0 liter VW/Audi TFSI 4 cylinder engines (I used to work in an Audi Parts Dept).