Why would you choose demak over honda and yamaha? If you want best value for money I would suggest you take Modenas MR1, proven reliability and suit your budget, RM3200 OTR.

1) Spare parts? Which well known motors fits? or does it have its own?
Yamaha spare parts can fit into this model, I guess no issue for finding spare parts.

2) Durability? Performance? Reliability?
I've been hearing china made motorcycles quality are really bad, first 10,000 km should be no problem. After warranty many problems would surface. But some said china bikes nowadays have improved a lot, just a question you dare not to try...

3) Long distance performance?
For a new bike, i don't think it has problem travel long distance. I can see a group of demak combat kaki travel long distance around peninsula malaysia.