Honestly mate your best bet is cash converters.
That Cannondale bike second hand is at best £200, the phone £140 so being generous £350. Nobody is going to sell a full-sus bike for £350. Unless it's one of them retro 2005 Y frame ones...
As for a downhill bike, you'll be lucky to get a half decent drivetrain for a downhill bike for £350 yet alone a full bike.
Maybe check facebook marketplace for a nicked Canyon Sender or YT Tues but nobody with any sanity would swap a dh rig for an old iphone and hardtail budget mountain bike.