I suppose the crank rings are welded, because the dude said it wasn't possible to change only one ring, and i should buy another crankset. How could i confirm this by myself?

The rear deraileur is a Shimano Altus. How do i know if it is compatible with 14-34?

If it is possible to change only crankset + chain + 14/34 freewheel, i can get away with less than 100€, and i think it's worth it!