Heater is not a big problem to change - it can all be done from the engine bay, few nuts and bolts, one electical connector, the wires that operate the speed/heat and the water pipes.
If you can get the whole heater you would be better getting that, saves you having to open up your one fit the matrix and riviting it all back together!
You might also find that the little pieces of pipe which takes the air from the bottom duct up to the window vents have fallen off and are warming the back of your dash, you can take the dash off on the drivers side and see this pipe to check it, and on the other side its behind the round grey cover which can be taken off.
Have you checked your thermostat isn't jammed shut meaning the engine never gets that hot?
Whats up with the water? You can change the jets easily if its them, if its the pump that can be changed easily too, might just need to clean out your tank and pipes, algae starts to grow in the lines and that block everything. I fixed them once by filling the system with thin bleach and letting it clean it all out, then disconnected the jets and just let the water shoot forward to clean all the bleach out the system, it gave them a new lease of life.
