Go feel stuff under the hood. Find out what gets hot to the touch and what doesn't. I've never worked on the 4-bangers, but this is general troubleshooting for any engine cooling system.
Hot radiator and cold heater core: lines disconnected or shut off, or air in the heater core. If gauge shows cold too, gauge screwed up. You're just not getting flow through your heater core. No telling what the prior owner could have done to the electrical system, so don't trust the gauges. He could have also bypassed the heater core due to a leak. Open the heater core box and look for a bypass pipe where the core should be.
Everything warmish but not hot: bad thermostat (I know you replaced it, but it's possible to get a new bad part), thermostat installed wrong(backwards), damaged thermostat housing. You're getting too much flow through your radiator and the system can't warm up.
Extremely hot engine, cold radiator and thermostat housing: failed water pump, system blockage, or very low water. Do a compression check. Your engine has overheated due to the cooling system not working at all. If it's because of air in it, the temp gauge will show cold even while your engine is happily converting its oil into charcoal.
Everything seems the right temp under the hood: blend door not working.
